The ball sack tap is the deft movement of tapping one's hand, palm up, onto someone else's ball sack (with or without clothes on). This can be done by males, to reward one another for something well done.
This can be demonstrated in the movie SuperTroopers during the first meeting, when Mac gives a ball sack tap to the Rookie after he sings the song.
"Dude, did you just give me a ball sack tap?"
"I'll give you a ball sack tap if you finish that whole thing of syrup"