Prerequisites: Trained By A Master, Breath Control, and Meditation.
Body Control (HT/Very Hard) is the skill to affect involuntary bodily functions such as heart rate, blood flow, and digestion. (B181)
It is referenced in GURPS Martial Arts, p. 54, on replacing HT for resistance rolls (including Pressure Points or Hand of Death form Innate Attack or Chi powers) and as an medical care skill.
Sean Punch has commented on Body Control in the following posts to the SJG GURPS Forums:
- On it not replacing Chi powers 
- On using it as a defense against starvation