Sorry if this is TMI:
Since sensations, responses and the linguistics vary from one man to the next, any number of definitions are correct; and, I'd hope others share in this thread.
I'm not experienced in Human Anatomy and am 'snowed' by the chatter of my Dentist or Doctor during any exam. But, after a read in Wikipedia, -- 
-- will give it a try:
My "abdominal contractions" are steady increasing "flex" of my abdominal muscles (Rectus Abdominus and Obliquous Externus) along the anterior Axillary Line which, as it grows, causes me to assume a fetal position. In the extreme (as I sink into orgasm) my Pecs and Lats are also affected. That tends to pull my shoulders downward. Usually the contraction starts when I'm on my back and as it grows I roll onto my right side, assuming a semi-fetal position with my legs slightly bent.
Simultaneously there's a definite growth of 'warmth' in my Navel Chakra (Dan Tien) which expands upward into my Heart Chakra.
I think that gents, more into Yoga, Tantra and Reiki, would sense and describe this as an 'Energy Move.'
Now, the 'nutsey' part. I experience 'visualizations" ** during the onset and the build of most of my p-wave and orgasmic sensations. Over the last two years there have been periodic changes in what I "see / visualize" during the p-wave and orgasmic part of my Aneros Sessions. Starting on my back I 'view' the onset of the wave and contraction as a sheet of 8 x10" portrait paper (with a 1/4" white border). It first appears simultaneous with the onset of Chakra warmth and continues until I'm stable into a calm seas mode or into intense orgasm. (body position at that point is probably close to fetal but I'm too deeply involved that I cannot later recall anything about position or posture.) The mystique of the orgasm overwhelms this visual and during rare Super-Os there is just color and pattern and sometimes sound that is just an 'intense wad of plasma.' -- http://en.wikipedia.org/wiki/Plasma_effect --
Most of this summer, my out of body view (from outside my body) was of a flat sheet of portrait sized paper overlying my abs. the paper was a dark tan in the central are. This fall the color of the paper is a metallic tan and it is somewhat twisted with a wavy surface.
** Note: These visualizations seem to vary over time so I don't think they are typical Synesthesia.
hth... rook
