Originating from the StarCraft unit "The Lurker" this phrase has now come to be a euphemism for sexual intercourse or, more specifically, the act of penetration.
When fucking a girl in the vagina, and you in one sly, dastardly, and courageous movement you remove the penis from the vagina, and instantly slip it into her asshole.
*note* unless the girl has a wide girth...this move should follow the spit prep