When one pulls his pants down to expose bare bottocks, places them by an unsuspecting victim's head, and proceeds to slap his own bottocks as if he is playing the bongo drums. This results in an unpleasant yet humorous surprise for the victim.
In a movie theater: "Hey, you tap that guy in front of us on the shoulder while I give him the butt bongos". *Friend taps the person sitting in the row in front of him on the shoulder. The victim then turns around to see two naked bottocks in close proximity to his face, while two hands are slapping them in a rhythmic fashion. *