Referee's Signal Number 2 - Run (or Restart) the Game Clock
This signal is used by any official to indicate that the clock is not to stop - particularly if a ball becomes dead near the side-line. It is also used by the Referee when blowing the Ready-for-Play (S1) whistle to re-start the clock if it has been stopped to administer a foul, or to set the chains for a First Down or for a Referee's time-out.
Begin the motion with your arm at the bottom of the circle in front of your body and move it across your body and around above your head and out to the side and down to complete the circle. Don't be too vigourous, but don't dawdle. Repeat it two or three times.
If you are using this to indicate that the ball has become dead in-bounds near the sideline, accompany it with a loud blast on the whistle. It is appropriate to make this signal in this circumstance even when you are the on-field timekeeper, as it lets everyone else into the secret.
Make sure that you do not hit anyone with your hand (it hurts!).