It just empties out your bowels. You may walk around with a pound or two of food and/or fecal matter in the pipe waiting to come out at any given time. Normally you eat a bit every day, and defecate a bit too, keeping roughly the same amount inside of you. A colon cleanse just empties it out all at once. As soon as you eat again, the process goes back to normal, and you gain that pound or two back again.