To force the head back and to the left
When getting head your load is so powerful it forces the head to jerk back and to the left. "You JFKed her"
When a man perchs on the headboard of his bed, jerking off, trying to shoot a load on his significant other, while they walk around the bed waving to an imaginary crowd of people.
Man: Ugghh! Take that bitch!
Woman: Oh, I've just been JFKed!