The clock starts to count down immediately after a play is whistled dead, meaning the ball carrier has either been brought to the ground by a defender or a pass has fallen to the turf. The offense then has 40 seconds to hike the ball and begin the next play. Within the 40 seconds allotted, the offense must form a huddle, call a play and get in position prior to the ball being snapped. In order to preserve time, the offense occasionally elects to use a 'no huddle' strategy, in which two or more plays are called at one time to negate the need for a pre-snap conference.
Play clocks are designed to maintain a relatively fast-paced game. Without a play clock, offenses could take several minutes to get organized in between plays. If permitted to do so, games could take upwards of 5 hours to finish, which tends to be far less entertaining than the standard three hour timeframe fans have come to expect.
If the 40 second play clock expires before the offense can hike, a delay of game penalty is issued by the referees. A delay of game penalty results in the offense having to back up five yards from the previous spot before beginning the next play.