I believe the poop-effect is due to a few reasons (synephrine, caffeine, and black tea being the main culprits I know of):
- agonizing of the adrenoreceptors in your intestines will speed up the passing of crap.
- enlargement of the prostate makes your ass feel fuller.
- relaxation of sphincter muscle means ass pressure increases.
These are the effects I can think of offhand, at least. There may be others...
I personally like the effect. Taking T-rex or an EC stack after a day of high calorie eating makes me feel like I crap out a good 15 lbs.