I use a "double tweet" in my games. Could be because I learned it from the reffs willie speaks of. I think that a double tweet distingues that whistle blow from other whistles, so when my partner hears a double whistle, he won't put the ball into play, or he shouldn't. I dunno, take it for what it is worth, I am not sure if it is "correct", I doubt it is, but I think it helps my game.