the last lap of a race on a track, in which a gun is fired as the racer in first place completes the penultimate lap. this immediately causes a surge in adrenaline, and even though the racer is in more pain than most people can imagine, he or she immediately picks up the pace and goes balls to the wall.
"c'mon you can do it! the gun lap is all about balls to the wall! balls to the wall!"