I am not a doctor **warning** LOL
From what I have been reading. "Kegels" flex all of the muscles of the pelvic floor simultaneously to one degree or another. (The PC,BC, and IC) You may hear varying arguments on what it feels like to "isolate" one from the other, and there may be ways to "stress" or contract on of the three main muscle groups in varying intensities, depending on position, technique, or focus, but apparently from what I have read from doctors, the nerves which cause contraction in the pelvic floor, contract the PC,BC, and IC at the same time.
Furthermore, I can find no evidence or description of any muscles which work in the opposite direction of the pelvic floor muscles. In simple terms, there are no other muscles in the pelvic floor area which will stretch, the opposite of contraction, the PC,BC, or IC. Muscles do not have a "stretching" or "elongation" function. For example, the biceps can not stretch itself, an opposing muscle, in this case the triceps, must contract to stretch the relaxed biceps. Since there are no "triceps" analogous muscles in the pelvic floor area, to stretch them, (at least by using other muscle contractions and not by external force on the penis for example) another group of muscles outside of the pelvic muscles outside of the pelvic area must me used.
According to a couple of articles from doctor's that I have read, the only way to "stretch" or elongate the pelvic floor area by using muscular contraction is by contracting the abdominals. I am guessing that this contraction of the abdominals simply squeezes enough of the organs and intestines to place a "downward" force upon all of the muscles in the pelvic floor area. Like squeezing a tube of tooth paste :D
Anyway, a legend around here, recently casually stated that a reverse kegel may be like taking a crap. And that probably is the best and most simple explanation after all. Forcefully pushing out an imaginary large turd (or trying to pee very hard) may be the best description of a true "reverse kegel". But this muscular action certainly is not done by the PC, BC, or IC.
Hope that helps. I am sure someone else will chime in to say that I am a moron :D
Tom