Referee's Signal Number 3 - Stop the Game Clock
This signal is used to denote that the game-clock is to be stopped and may be made by any official. It will be used when the ball-carrier has run out of bounds or when a (team or official's) time-out is granted.
The hands should start above the head and should be moved to the "10-to-2" position and back - don't go any further if you are in a crowd as you risk hitting a player on the helmet with your hand (it hurts!). This should be repeated 3/4 times in an unhurried manner, and is the only signal that may, and indeed should, be repeated by the other officials. It may also be made whilst on the run.
If you are the covering official, it should be accompanied or preceded by a loud blast on the whistle, and if you are the on-field timekeeper you should stop the clock before starting the signal.