It stands for a "key-play" ...that's how it started. A kick-off or punt, or usually a special teams play is the type of "key-play" that could make a drastic swing in the game. Albeit field position, or a botched snap or catch, anything that can totally alter the flow of the game. And yes, only on "key-plays" do we do this. (Not because we don't have a cowbell, or it's a good luck thingy) We didn't invent it either. Other schools around the country have been performing the ritual every game as well.