Big Willie Style is a shock-style sex move. It is accomplished by first having regular intercourse between a girl and a guy, but right as the guy is about to cum, he pulls out, plunges it into the girl's ass, and unleashes his load while singing "Na na na na na na na - gettin jiggy wid it," the classic hit by Will Smith. A greater effect is added if the guy raises the roof while doing this.
Jim: What'd you end up doing last night?
Paul: We had sex, but it was boring so I unveiled the new Big Willie Style move.
Jim: That's epic, man.