When you screw someone up the anal, or in other words, have butt sex. When you call someone an anal screw, you're calling them gay. It's also used as an expression: "Anal Screw!". When you say this, your saying "this is gay".
You're getting stuff out of your locker, reach down for a book, come up, hit your head on a metal shelf inside your locker and exclaim, "Anal Screw!".