Warning: This will be a somewhat complicated blog. Those who don't like long explanations may want to leave.
Ok, so I have been doing a little thinking, specifically about Luffy's Gears and how they work. Here goes......
Alright, as we all seem to know, Gear Second involves using Luffy's legs as a blood pump to circulate oxygen more quickly, allowing greater use of energy within the muscles (Hence, more strength, as the muscles can expend more energy).
The downside of this wonderful ability is that it uses up the energy sources much faster. Through cellular respiration, food molecules are converted into energetic molecules through a complicated process. However, it requires lots of oxygen, and food. When Luffy uses Gear Second, he burns through this supply of oxygen and food much faster, generated a large amount of energy for a short time.
We have seen Luffy use Gear Second to its full abilities, especially with Soru. Instead of kicking off of the ground many times to increase speed, Luffy uses his elastic body to propel himself forward (I assume this takes a great deal of strength to rapidly move his legs in such a manner). He also increases the speed of his attacks, as the new burst of energy allows him to apply more force, and therefore speed, to his attacks.
Now here are my questions. How can Luffy apply Gear Second to just one part of his body? If Luffy was strong enough, could he use the Soru technique without Gear Second? And what about that smoke? Is it just excess oxygen (highly unlikely) or is it like sweat evaporating on his skin?
One final thing on this gear. As Lucci states, Luffy's stamina is not able to keep up with the rapid energy burst. Therefore, through physical conditioning (which involves training your cells to store more oxygen), couldn't Luffy improve the amount of time in Gear Second?
I have a feeling that I am somehow completely wrong in my reasoning, so if I am, I would appreciate someone telling me. I was gonna write some stuff on Gear Third, but this blog is long already.....