The play clock times each and every play during the game. If the kicking team does not kickoff prior to the play clock expiring, they will be assessed a delay of kickoff penalty. It is also known as a delay of game penalty. The term could also be used to describe the delay instituted on the 2nd slate of games on Sunday if games from the 1st slate are running over their allotted broadcast time.
On a kickoff, the play clock begins to tick down from 40 seconds as soon as the head umpire blows the whistle and signals for the clock to start (the game clock begins at precisely the same time). A delay of game on a kickoff is extremely rare, as most teams kick the ball within a few seconds of the whistle being blown. There may be some cases where a team purposely takes the delay of kickoff penalty to waste time, or to give their kicker more space in which to kick the ball.