A sexual move whereby during doggy style sex the penis, in one swift move, is removed from the vagina and reintroduced into the anus (also known as changing at baker street). The shock causes the recipient's head to rear up at which point the gentleman smacks it down with his fist as one would a miscreant pack animal. Hilarity ensues for all involved. Except possibly for the "mule-ee".
Dave: The girlfriend dumped me last night, mate.
Steve: Really, mate?
Dave: Yep. Tried the mule. She wasn't best pleased.