the act of thrusting the hips back and forth while naked causing the genitals to swing like a pendulum alternately slapping the stomach and butt.
Usually performed by males though some females with ample piss flaps
are capable of this exercise while some males regrettably, are incapable.
Mark broke the ice when he did the stomach to butt and got the party started.
I was shocked when Mary's mother did the stomach to butt at the wedding reception , her wizard sleeve
looked like balls swinging.