Whilst going down on a girl, usually a one night stand, not a girlfriend, you proceed to bite down on the inside of her vagina, and never let go. It is customary to thrash your head back and forth as you do this, as a shark would do with it's prey.
Girl 1: Oh, shit! Why's your vag bleeding so bad sweetie?
Girl 2: This fucker gave me a crippling shark last night.