Did you hit her hard enough?
Donkey Punch
Banging a girl doggy style and then moments before you cum, sticking your dick in her ass, and then punching her in the back of the neck. The blow to the neck will make her arc her back and tense her arse muscles, which will constrict the Penis and give you a tremendous orgasmic experience when you ejaculate, but for it to work correctly, the girl must be knocked out so that her asshole tightens up.