Seimei Kikan is a technique used by the CP9 members Kumadori and Rob Lucci. It allows the user to control their bodily functions. FUNimation calls it Life Return, while in the FUNimation-dubbed Unlimited Adventure, this technique is called Rebirth. When Chopper saw it in action, he mentioned a similar ability called Bio Feedback.
At the sea for a thousand years, and at the mountains for a thousand years, breathing the wind and drinking pure water, he passed the training of a Sennin. Enduring it all, he gained the Life Return. Humans are able to, from the hair on their head to their intestines, their toes, even to their most minute hairs, to concentrate their senses and stretch them to their limits, making it possible to control every part of their bodies.
Kumadori demonstrates this when he was able to eat all the food within a giant refrigerator, digest all the food and most of his body fat, turning him super-skinny, then absorb all the nutrients in the food turning him back to his original size. He also uses Seimei Kikan to use his Hair Bind, in which he controls his hair to trap his opponents or form hands with which he performs Shigan.
Somehow, Rob Lucci also knew and utilized Life Return. He only used it in conjunction with Kami-e to make his half-leopard form smaller and more maneuverable. It is unknown how Lucci was able to use Life Return or if he has the same level of mastery as Kumadori.
Kami Shibari (髪縛り,Kami Shibari?, literally meaning "Hair Bind"): Kumadori lengthens his hair and manipulates it to bind his opponents. In the Viz Manga and Funimation dub, this is called Hair Bind. The term for this is a slight play on the term "kanashibari" (金縛り, lit. "gold binding"), which either is a term for sleep paralysis or "being tied down with money".
Haru Gin Jo (春・吟・情,Haru Gin Jō?, literally meaning "Spring Poem of Passion"): After catching his opponent in Kami Shibari, Kumadori aims his staff at his opponent for a finishing blow. He attempted this at Nami, but she slipped out of her jacket at the last second. In the Funimation dub, this is called Song of Spring Feeling.
Yana Gin Jo (柳・吟・情,Yana Gin Jō?, literally meaning "Willow Poem of Passion"): Same as Haru Gin Jo, aside from the name. He attempted this at Chopper. In the Funimation dub, this is called Song of the Willow.
Shoka, Kyushu (消化 吸収,Shōka, Kyūshū?, literally meaning "Digestion, Absorption"): Kumadori uses this technique to return to his normal size body fat after eating all the food in the refrigerator in which Chopper had locked him in. In the Viz Manga this is called Digest, Absorb.
Shishi Shigan (獅子指銃,Shishi Shigan?, literally meaning "Lion Finger Gun): Rather than using his own fingers for Shigan, Kumadori shapes his hair into the form of hands and repeatedly uses the fingers of said hands to rapidly jab the opponent, similar to Lucci's Shigan "Madara", but less powerful. In the Viz Manga, this is called Shishi Finger Pistol.
Seimei Kikan: Kami-e Bushin (生命帰還 紙絵武身,Seimei Kikan: Kami-e Bushin?, literally meaning "Life/Existence Return: Paper Drawing Martial Body"): Using Seimei Kikan, Lucci makes his half-leopard form smaller, lighter, and possibly increasing his speed. At any rate, he is much more agile in this form, and harder to hit - as opposed to his hulking normal half-leopard form. In One Piece: Unlimited Adventure (dubbed by FUNimation), this is called Rebirth: Sketch Form, and in the Viz manga, this is called Life Return Paper Art Fighting Form.
Seimei Kikan: Kaijo (生命帰還 解除,Seimei Kikan: Kaijo?, literally meaning "Life/Existence Return: Cancellation"): Lucci uses this while in his smaller half-leopard form. It cancels his Seimei Kikan: Kami-e Bushin technique, and returns to his larger, stronger half-leopard form. in the Viz Manga, this is called Life Return Release. In the Funimation dub, this is called Life Return: Terminate.
Despite Kumadori's description of the Seimei Kikan implying that it takes 2000 years to master this technique, it is an exaggeration, as he was only 34 at the time, while Lucci was 28.
Other characters within the series have shown the capability of controlling their bodily functions similar to Seimei Kikan. Boa Sandersonia and Boa Marigold can both manipulate their hair into snake-like creatures; Speed Jiru appeared as a plump person at times, but slim at others. It is unknown if Jiru's abilities stem from Devil Fruit powers or not, but the two Gorgon Sisters already have Devil Fruits that allow them to transform into snakes, so it is possible they are using something similar (if not the same) as Seimei Kikan.
Also similar to the muscle size manipulation of Seimei Kikan, Lao G has showcased the non-Devil Fruit-related ability to recreate the strength he used to possess during his youth, giving the old man an exceedingly bulky physique (as opposed to his usual, frail-looking body).
For some reason in the anime, when Tony Tony Chopper roared in Monster Point, Kumadori could not control his hair temporarily and it froze where it was before returning to him; it was as though the roar frightened Kumadori or even the hair itself. Alternatively, he became distracted and subsequently his control failed.