Checking back, a year later, as I see some people know exactly the feeling I'm talking about.
I have however found nothing conclusive, only speculations. Some think it's like a voluntary release of adrenaline, or manipulating the nervous system etc. I found one post that claimed to have ran some tests aswell:
"I have been practicing western medicine for 30 years and for a change also just completed my masters in Oriental Medicine (2014). [We] have for years used different equipment to play with my "ability" while in the ER/OR/Office while no patients: PulseOX, EEG, EKG, spirometry.The fact is, it is an instantaneous- literally as fast as thought phenomena/ability that can send my HR up to 200bpm is seconds, all while maintaining a NSR-normal sinus rhythm (PQRST remains unremarkable, otherwise); it is simple tachycardia. The pulse ox remains saturated and whereas the BP may raise to over 160/96 within seconds, the arterial pressure at rest does not substantially rise.I have always asserted (the reason is lost with age) that I "squeeze" my adrenal glands. It does not matter that western medicine says that this is not possible. What matters is the similar reports from disparate people all around the world. We are all describing something similar. Variously, some report that they are squeezing something in their head. Perhaps this is due to the incredible electric-like sensation tingling around the neck, head, and shoulders, pupils changing size, regardless of light conditions.My breathing does not change and while my tidal volume remains the same we once calculated that my stroke volume for cardiac output only begins to become effected at x ___bpm after a few minutes. Those of us who do this cannot really do this for minutes at a time; we exhaust then stop.I often would engage in conversation, laugh, or drink water just to demonstrate to others that this is not contemplative, or bearing down on pelvis, contra stimulation of parasympathetic response, etc. In every single case I have convinced my peers, professors, cardiologists, and others that this was real. In every case they stood corrected, surprised, but did not understand.So reserve some mystery for the unknown; these posters here are not unbalanced. We can tell, as a bona fides, others who have this versus others who are imagining it. It requires no thought, emotion, respiration, or other manipulation and it does not respond to time; its instantaneous. Oh, and when done, there will be an appreciable difference in armpit smell. This may be due to the secretion of porcine/like glands related to... fight or flight- sympathetic response!"
Anyways it seems interesting thing to study for someone working in the medical field, since there seem to be plenty of people who can do it and since it's so easily controlled running tests etc would not be complicated.