National Football League (NFL) teams are allowed to challenge plays when that team's head coach feels like the ruling on the field by the referee was in error. This is known as the instant replay rule found in the NFL rule book.
When the head coach wants to challenge a referee's call he must throw a red flag onto the field of play. When the referee sees the flag he will blow the his whistle and stop play in order to review the play in question.
What Can be Challenged?
A coach can challenge a number of plays which normally involve possession of the ball. For example, if a player is ruled to be out of bounds when receiving a ball but the coach feels differently, he may throw a red flag. Another popular time to throw the red flag would be when there is a question about whether a player has lost possession or fumbled the ball. Because challenges are limited, coached normally throw their flag only during critical times in the game.
Number of Challenges
A coach is allowed two challenges per game and can only throw the red flag if they have at least one of their three timeouts remaining for that half of play. A red flag may not be thrown if the team has no timeouts. If a coach is successful on both of his challenges, he will be awarded an additional challenge. If a challenge is unsuccessful, the team challenging the play is charged a timeout.
Last Two Minutes
Game officials will automatically review controversial plays during the last two minutes of each half of play. During this time the coach is not allowed to throw the red challenge flag.
Teams can not challenge a penalty call on the field. However, if a head coach throws their red flag and does not have a challenge or timeout remaining, his team will be penalized 15 yards.
When a challenge flag is thrown, the referee will stop play and go to a monitor provided outside of the field of play. They will look at slow motion replays at different angles before making a decision. There must be indisputable evidence for a referee to overturn a call. Although rules state the referee has one minute to do this it often takes longer.