When a male is having sex with a female in the doggy position, and the male is about to cum he sticks his finger up her but and when she turns around to ask what the fuck he quickly pulls it out and cums in her eye.
When you surprise someone with a variation on a Wet Willy ... but instead of wetting your finger with your mouth, you wet it with your butt. And instead of putting it in their ear, you put it in their mouth.