Chief Brody
You start by inserting your finger inside an asshole (yours or hers) or a vagina. You then place one hand on top of your head, fingers extended as if to make a sharks dorsal fin. Then while humming that impending doom Jaws theme you maneuver your finger back and forth to swim like a fish through the water. Right before you blast the victim across their upper lip with your finger, you stop and say, "SMILE YOU SON OF A BITCH."
She was talking shit to her friends at the party about our sexcapades, so I gave her the Chief Brody.