Just found this article from ABC News containing a bit of info. Reading about the "why" of it is one thing, but I still cannot fathom why boys would actually do this, though.
That being said, after being aghast at reading about this "game," I did ask my middle child if he had ever heard of this before. He told me that one of his classmates had done it to him once and my boy had gotten very angry with the classmate, told him that he'd better not try that again. Classmate hasn't done it again. I hope that is the end of it and that neither of my sons encounter this "game" again.
'Sack Tapping' -- A Schoolyard Game Turned Ugly
Doctors Report Growing Fad: Boys Hit Each Other in Genitals, Post Video Online
Gone are the days when bullies merely gave wedgies. The newest fad in schoolyard violence? A quick slap to the testicles.
In the game of "sack tapping" or "ball tapping," as it is called, a boy will try to slap or flick the groin of another boy to inflict discomfort or pain.
Entire article: abcnews.go.com/print?id=10762563