When You are watching pornography, when the Male is "inside" of the female (or other male) While On his back, The small area of this butt that you can see underneath the ballsack is called the ass patch.
Dude Number 1 : Dude ! did you see that porn where the guy is doing that chick while on his back !?
Dude number 2 : Hell yeah I did ! He was SO showing off his ass patch.