Nantucket Ninja
If you're fucking your woman from behind, right as you're about to cum you karate chop the back of her neck to render her unconscious and thereby causing a contraction of her vagina which adds to your pleasure. The key to yell "Hiya!" right at the point you administer the karate chop.
Dude, I gave your sister the Nantucket Ninja last night and not only did I have the best orgasm of my life, but she doesn't remember a thing!