Powershot in Left 4 Dead is the ability to empty an entire Uzi or Assault Rifle clip in one fell swoop with this cheat. Included a video as well.
Pick up your weapon and switch to the pistol with Y. As you do this, hold down the left MELEE trigger. While meleeing, switch back with Y and press the right SHOOT trigger at the same time. You should be holding all three buttons: left MELEE, right SHOOT and the Y button. Now, wait a few seconds (ideally 10), and release the left MELEE trigger to unload your entire clip. Usually frowned upon in Versus matches.
Tags: glitch, Left 4 Dead, powershot, Xbox 360