Running out the clock
In American football, each quarter of a game is measured with a 15-minute game clock, or 12-minute clock in many high school football codes and the German Football League. A team in possession of the lead and the ball will attempt to use up as much of the game clock as possible in order to bring the game to an end more quickly, thus denying the opposition another chance on offense.
Time on
Each quarter has a specific length of playing time, which can vary in different forms of the game, but at senior level is usually 20 minutes. When the umpire stops play for a score, injury, the blood rule, to award a 50-metre penalty or to reset play for a mark or free kick, he raises one hand above his head and blows his whistle; this is called "blowing time off". This tells the timekeeper to stop his clock and stop counting down playing time. When the umpire again raises his hand and blows his whistle, called "blowing time on", or when the ball is bounced or thrown in, the timekeeper starts his clock again.