it's called prostate stimulation...
Scientifically speaking, 1/3 of the male penis is inside...meaning you cant see it...and there are a lot of nerves in the anal region....scientists say it can be very stimulating and enjoyable to males and females
hence the phrase: prostate massage or prostate stimulation...google it
btw, for the record, that's totally gheigh and i'd never try it, but hey it works for a much larger portion of non-gay men