Number of Challenges
NFL challenge rules allow for two challenges for each team during a game. A third challenge can be awarded to a team only if they are successful in both of their previous challenges. If one or both of those challenges is denied, then the team does not get a third challenge. In the last two minutes of each half of play, team challenges are not necessary. The officials automatically review all controversial plays during this time.
NFL challenge rules state that the only person who can issue a challenge is the head coach of the team. The head coach does this with a special red flag that he keeps with him throughout the game. The head coach signals that he wants to challenge a play by throwing the red flag onto the field of play. When the officials see the red flag, they blow the whistle and stop the clock. This must be done before the ball is snapped on the next play. If the ball is snapped, then a challenge cannot be made.
What Can Be Challenged
Not every play can be challenged under NFL challenge rules. Teams cannot contest a penalty call or the lack of a penalty call, even if the blown call is obvious. Most challenges involve the possession of the ball, whether a player is down or the spot of the ball. Since the number of challenges is limited, head coaches usually save their challenges for critical times during a game. A turnover or third down conversion can determine the winner of a game. Instant replay can reveal things that happen in a fraction of a second or within a fraction of a yard. Other plays not eligible for challenge are a score for either team, an interception, a fumble or backward pass recovered by an opponent or that goes out of bounds through an opponent's end zone or a scrimmage kick that is recovered by the kicking team. If a team throws a flag for a play ineligible for a challenge, it will be charged a timeout.
When a challenge flag is thrown, the referee goes to the sidelines and watches replays of the challenged play on a monitor. The referee can watch all the angles that the television cameras captured. He can watch the play in slow motion, backward, forward and in still frame. NFL challenge rules state that the referee has one minute to review the challenged play, but this sometimes takes longer. The referee must see "indisputable visual evidence" that the call on the field was incorrect. In cases where the replay is too close to tell, the call on the field stands.
According to NFL challenge rules, a team can only issue a challenge if it has a remaining time out. If the team does not have any more time outs, then it does not matter that the head coach still has a challenge. If a team has run out of timeouts in the first half, they can save their challenges for the second half, when they get more timeouts. A team needs a timeout to call a challenge because the team is penalized a timeout if its challenge is unsuccessful. If the challenge is successful, then the timeout remains for a future use. If a team calls for a challenge when it has none left, it will be docked a timeout.