The prostate is the organ responsible for producing a good part of a man's ejaculate. Located inside the body, in the region between the anus and the base of the scrotum, the prostate is primarily accessable through the anus. Inserting a finger into a man's anus up to about the second knuckle (as you are facing him), then curling the finger slightly towards yourself will give you direct contact and stimulation of most men's prostate gland.

You'll find that as a man ejaculates, the entire area around the prostate, including the anus (especially if you have your finger in it), will throb noticeably. Stimulating this area even lightly during orgasm causes a man to have uncontrollable orgasmic spasms. Often, men will experience much longer periods of orgasm and will ejaculate with far greater intensity and volume.

This phenomenon caused the creation and popularity of the butt plug, a cone shaped plug that is inserted into a man's anus during sex and left in position until orgasm.