When a woman is riding a man in the cowgirl position, she reaches around her back and smacks the unsuspecting victim in his balls. The man then proceeds to buck like an angry bull. If the woman can hang on for 8 seconds, this is considered a full ride.
"So my girlfriend is going to town on me last night cowgirl style and pulls a Bullrider. She didn't make the full 8 seconds so I Donkey Punched her in the grill"