This technique involves Luffy speeding up the blood flow in all or selected body parts, in order to provide them with more oxygen and nutrients. This makes him much faster and more powerful. The technique uses even more oxygen and energy, using up more food and causing Luffy to pant heavily. The increase in his metabolism causes him to be hungrier than usual or even have a paralysis type effect after prolonged use of it, like in the second fight with Rob Lucci, where Luffy was only able to lie on the floor. Lucci, one of the first of Luffy's opponents to witness it, compares the process to doping. Doping is adding more red blood cells to the body part/s, which in turn allows for greater delivery of oxygen and nutrients to the body part/s. In Luffy's case, his organs and blood vessels are made of rubber allowing them to expand and contract at greater rates without tearing. Essentially Luffy increases the blood flow by forcing more blood through his blood vessels thus increase his fighting ability. This mode's enhancements to Luffy's physiology giving him the ability to use a new, stronger set of techniques which are generally the same as his old ones, but are so fast that even a skilled assassin would have a hard time actually seeing them. He developed this ability after encountering CP9, and watching them use Soru. This is called Gear Two in the Viz Manga, Second Gear in the Official Simulcast subs and in the FUNimation Dub.
I've thought of a way to fight with all my strength so I won't lose to anybody! So I won't ever have to lose anybody you won't be able to catch up with me anymore. All of my techniques have evolved by one step.
— Luffy's determined answer to Blueno in their fight
Prior to the timeskip, Luffy would initiate this technique by using his leg as a pump to increase the blood flow in all of his body parts. As a result, all of his body parts become stronger and faster. The technique used more oxygen and energy, using more food and caused him to pant heavily during use. He has been able to heavily damage some of the strongest individuals in the series whilst this technique was active, such as Magellan and Blackbeard. When this technique was in effect, his entire metabolic rate went so high that his sweat vaporized rapidly making his body look as if it were steaming and, in the anime, his entire skin turns slightly pink and shiny from the increased blood flow. He also says almost all of his attacks in English as opposed to Japanese.
At first, Luffy could only use Gear Second for a short period of time, because the accelerated speed at which nutrients are used up depletes his body rapidly. The mechanics behind this enhancement thus also mean reduced strength and agility after prolonged use, as pointed out by Lucci when Luffy's attacks start losing their speedy edge because with all his nutrients running low, having a faster blood flow would be useless as there are no nutrients to use. Due to this, as also pointed out by Rob Lucci, Gear Second hacks away at Luffy's life force dramatically, increasing a chance of having a much shorter lifespan because he is starving his body, though Luffy rebukes it, saying that it does not matter as long as he saves his friends.
After Gear Second wears off, Luffy can recover quickly by replenishing his body's nutrients by eating meat. Later in the Impel Down Arc, Luffy demonstrated a more excessive use of his Gear Second ability without such exhausting costs like his fight with Rob Lucci, as well as being able to activate it at conveniently brief periods of time to save his stamina and strength reserves for longer fights. This is possibly because of Luffy's mastery over the form along with his desire to save Ace, and possibly the large amount of food he ate in Newkama Land at Impel Down.
While not in the anime and manga, in the 7th movie of the series which takes place before the Water 7 arc, Luffy briefly and unknowingly activated Gear Second during his fight against Ratchet, the antagonist of the film, when the sheer pressure of the machine Luffy is forced to hold up unknowingly causes his legs to end up pumping his blood flow. He then defeated Ratchet with a Gomu Gomu no Jet Bazooka, although he thought that it was a regular Gomu Gomu no Bazooka and was even surprised at the amount of force it had produced.
After the timeskip, it appears that Luffy has refined his control over this technique as he has now been shown capable of activating it by pumping blood in only one particular part of his body: the trigger works very fast as he is capable of activating Gear Second in an instant and does not even have to assume his usual stance to initiate it. Thanks to this new found control over his technique, Luffy now has the ability to select which body parts Gear Second will be active in, instead of having to activate it in his entire body; though the anime depicts this as a particular body part undergoing the pumping motion, such as his arm.
To perform any actions with this Gear, Luffy can now initiate Gear Second in the part of his body that he wishes to use and deactivates it once he has accomplished the task. Thus Luffy is now able to use Gear Second for his usual fighting style without using it on his entire body. Although the anime still has his entire body undergoing Gear Second's effects, but it still only lasts very short periods of time. With this new mastery over Gear Second, combined with Busoshoku Haki, he can now easily destroy a Pacifista and take down a fishman as large as Wadatsumi with a single "Gomu Gomu no Jet Pistol". Furthermore, using a combination of Haki and Gear Second, Luffy is somehow able to ignite his limbs to perform even more devastating and explosive attacks.
Most of the techniques in this mode are named the same as the regular attacks with "Jet" added after the "Gomu Gomu no" prefix, though often times, Luffy will omit this prefix when using several attacks in succession. The known named techniques that are used by Luffy so far in this form are as follows:
Gomu Gomu no Jet Pistol (ゴムゴムのJET銃（ジェットピストル）,Gomu Gomu no Jetto Pistoru?, literally meaning "Rubber Rubber Jet Pistol"): Luffy delivers a Gomu Gomu no Pistol so fast that the fist's movement is invisible, and it truly seems like he shoots his opponent, except that the resulting impact is not akin to an actual pistol, but to a small hand-held cannon and powerful enough to propel the unlucky recepient several meters away. The incredible speed both makes the strike far more powerful, and harder to dodge. When using this technique, he points his other hand out to aim at his target. In the Viz Manga and FUNimation dub, this is called Gum-Gum Jet Pistol. This was first seen being used against Blueno.
Gomu Gomu no Jet Stamp (ゴムゴムのJET（ジェット）スタンプ,Gomu Gomu no Jetto Sutampu?, literally meaning "Rubber Rubber Jet Stamp"): Luffy delivers a Gomu Gomu no Stamp, with the same effect as the Jet Pistol. In the Viz Manga, this is called Gum-Gum Jet Stamp. This was first seen being used against Blueno.
Gomu Gomu no Jet Bazooka (ゴムゴムのJET（ジェット）バズーカ,Gomu Gomu no Jetto Bazūka?, literally meaning "Rubber Rubber Jet Bazooka"): Luffy charges at his opponent at high speed, and delivers a incredibly fast and strong Gomu Gomu no Bazooka. In the Viz Manga, FUNimation dub, and FUNimation subs, this is called Gum-Gum Jet Bazooka. This was first seen being used against Blueno. It was apparently powerful enough to break through Blueno's strongest Tekkai technique, Tekkai: Gou, as well as Boa Marigold's Busoshoku-enforced guard. During the battle against Moria in his Shadow's Asgard form, Luffy demonstrated being able to use this move to propel himself from mid-air, allowing him to perform this attack continuously.
Gomu Gomu no Jet Whip (ゴムゴムのJET鞭（ジェットウィップ）,Gomu Gomu no Jetto Wippu?, literally meaning "Rubber Rubber Jet Whip"): Luffy delivers a Gomu Gomu no Muchi, so fast, that only the slipstream is visible. In the Viz Manga and FUNimation dub, this is called Gum-Gum Jet Whip. This was first seen being used against Rob Lucci in order to protect Franky from him.
Gomu Gomu no Jet Bullet (ゴムゴムのJET銃弾（ジェットブレット）,Gomu Gomu no Jetto Bureto?, literally meaning "Rubber Rubber Jet Bullet"): An extremely fast, close-range punch with very great force. When attacking head-on, the stretching of the original Bullet along with the resulting snapback seem to occur almost simultaneously at once, making it virtually inescapable. This was first seen being used against Rob Lucci. In the Viz Manga and FUNimation dub, this is called Gum-Gum Jet Bullet.
Gomu Gomu no Twin Jet Pistol (ゴムゴムのツインJET銃（ジェットピストル）,Gomu Gomu no Tsuin Jetto Pistoru?, literally meaning "Rubber Rubber Twin Jet Pistol"): Luffy punches with both arms simultaneously. Visually, the move only differs from his normal Gomu Gomu no Bazooka in that instead of doing an open palm strike, he has clenched fists which leave two knuckle shaped imprints on his foe. As since the move hit so fast, the air from it really looks like Luffy is literally firing two pistols. This was first seen being used against Rob Lucci and is one of the few moves unique to Gear Second (unless the anime-only regular "Gomu Gomu no Twin Pistol" is included). In the Viz Manga, this is called Gum-Gum Twin Jet Pistol.
Gomu Gomu no Jet Gatling (ゴムゴムのJET銃乱打（ジェットガトリング）,Gomu Gomu no Jetto Gatoringu?, literally meaning "Rubber Rubber Jet Gatling"): This is Luffy's strongest technique in Gear Second. Luffy stands still, bends forward and moves his arms upwards at speeds which they can no longer be seen, leaving only jet streams in their place, very much like how the Jet Pistol looks. This results in an effect comparable to Foxy's Kyuubi Rush, except that Luffy does not need to make use of Noro Noro photons to slow down his target in order to accumulate the hits. The hits land so fast that, after a volley, they seemingly connect simultaneously. The circles of air bursts from the punches surrounding Luffy, resemble the many barrels of an actual Gatling gun. However, due to the large amount of energy it uses, Luffy cannot sustain it for long, although the time it is used is longer than the regular Gatling. During his fight against the combined efforts of Marigold and Sandersonia, Luffy demonstrated an omnidirectional version of this move by falling on his back, then, while lying down, launches a rapid barrage of powerful high-speed punches from the ground, effectively nullifying both sister's attacks. This was first used to defeat Rob Lucci. In the Viz Manga and FUNimation dub, this is called Gum-Gum Jet Gatling.
Gomu Gomu no Jet Rocket (ゴムゴムのJET（ジェット）ロケット,Gomu Gomu no Jetto Roketto?, literally meaning "Rubber Rubber Jet Rocket"): A Gear Second version of Luffy's Gomu Gomu no Rocket technique. After grabbing onto something with stretched out arms, Luffy instantly propels himself forward with blinding speed. This was first seen being used against Moriah. In the Viz Manga, this is called Gum-Gum Jet Rocket.
Gomu Gomu no Jet Migawari (ゴムゴムのJET（ジェット）身代り,Gomu Gomu no Jetto Migawari?, literally meaning "Rubber Rubber Jet Substitute"): Somewhat of a gag move where Luffy stretches his arms to grab a person (an ally, an opponent, or otherwise a simple bystander) from afar then pulls them close to where he is to block an attack that Luffy would otherwise be incapable of surviving on his own. Using his Gear Second's speed, Luffy can capture himself a shield very quickly. He has a normal (non-Gear Second) variant of this move where Luffy sends the recipient spinning towards the assaulter to preoccupy them as he makes his escape. This was first seen being used by Luffy on Buggy, using the latter as a human shield against Dracule Mihawk. This is called Gum-Gum Human Shield Jet in the Viz Manga and Gum-Gum Jet Human Shield in the FUNimation subs.
Gomu Gomu no Jet Lance (ゴムゴムのJET槍（ジェットランス）,Gomu Gomu no Jetto Ransu?, literally meaning "Rubber Rubber Jet Lance"): A Gear Second technique that first appeared in the anime, however time after, Luffy makes the same move with its official name. Luffy delivers a very fast Gomu Gomu no Yari that seems to connect instantly with his opponent. This was first seen in the anime being used against Blueno.
Gomu Gomu no Jet Missile (ゴムゴムのJET（ジェット）ミサイル,Gomu Gomu no Jetto Misairu?, literally meaning "Rubber Rubber Jet Missile"): A Gear Second technique that first appeared in the anime. Luffy stretches and grabs onto an enemy's body. Then, he very quickly retracts, crashing into the foe. This was first seen in the anime being used against Blueno.
Gomu Gomu no Jet Axe (ゴムゴムのJET斧（ジェットアックス）,Gomu Gomu no Jetto Akkusu?, literally meaning "Rubber Rubber Jet Axe"): A Gear Second technique that first appeared in the anime. Luffy stretches his leg vertically and then crashes it down on his opponent, unlike the usual version however, rather than merely smashing his foot into the enemy the speed at which it is done is so quick that rather than attacking the area directly in front of him, his Jet Axe strikes several meters in length, thus increasing its range. This was first seen in the anime being used against Lucci.
Gomu Gomu no Jet Rifle (ゴムゴムのJET回転弾（ジェットライフル）,Gomu Gomu no Jetto Raifuru?, literally meaning "Rubber Rubber Jet Spinning Bullet"): A Gear Second version of Luffy's Gomu Gomu no Rifle technique that first appeared in the anime. The winding of the original Rifle along with its snapback happening almost at the same time as Luffy speeds towards the target makes this move impossible to evade. This was first seen in the anime being used against Blueno.
Gomu Gomu no Jet Hammer (ゴムゴムのJETハンマー,Gomu Gomu no Jetto Hanmā?, literally meaning "Rubber Rubber Jet Hammer"): The Gear Second version of Gomu Gomu no Ozuchi, which was not used since Luffy's battle with Don Krieg. Luffy wraps his legs around his opponent and, at an extreme speed, smashes him into the ground. Unlike the normal version, Luffy does not twist, and then un-twist his legs during the execution of this technique. Although in the anime he twists his legs but does not untwist them. It was first used against Vander Decken IX while Luffy was tied up. In the Viz Manga and FUNimation subs, this is called Gum Gum Jet Hammer.
Gomu Gomu no Jet Mace (ゴムゴムのJET鉾頭棒（ジェットメイス）,Gomu Gomu no Jetto Meisu?, literally meaning "Rubber Rubber Jet Mace"): Luffy kicks his opponent with extreme speed while falling and spinning. This was first used against Caesar Clown. In the Viz Manga, this is called Gum-Gum Jet Mace.
Gomu Gomu no Jet Spear (ゴムゴムのJET槍（ジェットスピア）,Gomu Gomu no Jetto Supia?, literally meaning "Rubber Rubber Jet Spear"): Simply a more powerful, Gear Second version of Gomu Gomu no Yari. It was first seen being used on the floor in Luffy's fight against Monet.
Gomu Gomu no Red Hawk (ゴムゴムの火拳銃（レッドホーク）,Gomu Gomu no Reddo Hōku?, literally meaning "Rubber Rubber Fire Pistol"): A Haki enhanced version of Gomu Gomu no Jet Bullet. After entering Gear Second, Luffy uses Busoshoku Haki to harden his arm. He then stretches it back like Gomu Gomu no Pistol. When cocked back, his arm ignites, releasing a stream of fire. When the attack hits, it creates a fiery explosion even underwater and deals immense damage. The Redhawk is a type of revolver made by the gun manufacturer Ruger, and the kanji name of this attack pays tribute to Ace'sHiken (火拳銃 means both "Fire Pistol" and "Fire Fist Gun"); the anime furthers this tribute by showing a flash of Ace when Luffy used this technique. This was first used against Hody Jones in his transformed state after the air bubble engulfed Noah. In FUNimation subs, this is called Gum-Gum Red Hawk, while in the Viz Manga it is called Gum-Gum Fire-Fist Pistol Red Hawk.
Buso Koka: Gomu Gomu no Jet Gatling (武装硬化 ゴムゴムのJET銃乱打 ,Busō Kōka: Gomu Gomu no Jetto Gatoringu?, literally meaning "Armament Hardening: Rubber Rubber Jet Gatling"): A Haki enhanced version of Gomu Gomu no Jet Gatling. Luffy uses Gomu Gomu no Jet Gatling while using Busoshoku: Koka on his arms to enhance the power of his punches. He first uses this against Caesar Clown, but it gets blocked by Monet using her Devil Fruitability to create a wall. In the anime, he uses this against Bilić, but misses due to Elizabeth's speed. In the FUNimation subs, this is called Armament Hardening: Gum-Gum Jet Gatling.
Gomu Gomu no Hawk Rifle (ゴムゴムの鷹ライフル（ホークライフル）,Gomu Gomu no Hōku Raifuru?): A Haki enhanced version of Gomu Gomu no Jet Rifle. After entering Gear Second, Luffy uses Busoshoku Haki to harden his arm. He twists his arm to create a spinning punch and increase the power. The first time this was used was against against Chinjao at Corrida Colosseum, and was sufficiently powerful to make Chinjao bleed, despite the fact that Chinjao was using Busoshoku Haki on his stomach to minimize the damage.
Gomu Gomu no Jet Bazooka (ゴムゴムのJET（ジェット）バズーカ,Gomu Gomu no Jetto Bazūka?, literally meaning "Rubber Rubber Jet Bazooka"): Luffy used the Gomu Gomu no Jet Bazooka with clenched fists instead of palms and imbues his arms with Busoshoku Haki. He first used this against Shuzo in the Z's Ambition Arc.