When you're banging a girl doggy style and she's about to climax, grab her by the ankles, stand her on her head and cram your fist down her dank exit hole, thus ruining her orgasm.
Also see Inverted Particle Accelerator for more sexually deviant partners.
Hey man, why is your girlfriend walking so funny?
I gave her the classic inverted show stopper last night and now her pelvis is out of alignment