A butt plug is a kind of dildo with a bulge in the center, much like a spindle, so that the shaft gradually thickens, flares out, and then begins to narrow again. In general, once inside the rectal cavity, the flare of a well- chosen buttplug will rub directly against those nerve clusters by the opening of the anus, the ones which signal your need to defecate. Depending upon how your brain is wired, denying or understanding this signal can be a very pleasurable experience, or an extremely uncomfortable one. Also, the 'tip' of the well-chosen buttplug will rub against a man's prostate when he moves, sending yet more signals of unknown quality to the brain. It's all in what you like.