When a male is bored in the shower and stretches his scrotum out and bounces his penis off of it so that looks like his penis is bouncing on a trampoline.
Guy 1. "You know when you're bored in the shower and you stretch your ballsack out and bounce your dick off of it like it's bouncing on a trampoline?"
Guy 2. "What the fuck??? No...."
Guy 3. "Oh, you mean playing the sack drum?"
Guy 1. "Yeah, exactly."