When someone with a tiny penis "try's" to masturbate. Their peter is so tiny that they end up just slapping their sack
Jessica: I caught Perry beating off in the bathroom yesterday.
Amanda: Trust me, his dick is so small he was just slapping the sack.
You would say this when a white male has sexual relations with a person of color
"Did you see James' new girlfriend? "
"yeah, he's been dippin' in the dark chocolate all of a sudden