At any time before the two-minute warning of each half or overtime period, the head coach of either team can signal a challenge by throwing a red flag onto the field. He's allowed only two challenges per game, but if both challenges are successful, he's given a third. He must issue his challenge before the next snap of the ball.

If the coach's challenge is successful -- that is, the ruling on the field is overturned -- then his team isn't charged with a timeout. If it's unsuccessful, it costs the team a timeout. A coach can't challenge any ruling if he doesn't have at least one timeout remaining.



The coach must consider his options carefully. Should he save a timeout so that he keeps the option to challenge? Should he risk a timeout on a challenge that may not be successful?

He must also decide quickly. He usually hasn't seen the play clearly from the sidelines. He may receive advice from an assistant coach in the booth who's watching the network television feed. Or he may heed his players who were close to the action. Knowing that the time to decide is limited, an offensive team receiving a favorable call might snap ball quickly to close off the possibility of a review.

Once the coach issues his challenge, the referee goes to one of two or three shielded television monitors near the field. He looks at the play from all relevant angles that are available. He has 60 seconds to review the images; then he must make a decision.

He may overrule and change the call on the field. In this case, he might make other revisions, such as resetting the clock.

He may confirm the original call, meaning that on replay he sees the call was correct.

He may rule that the original call stands, meaning he did not see enough visual evidence on replay to either confirm or overturn the call on the field.

During the 2009 season, there were 328 challenges -- 228 by coaches and 100 by replay assistants. A total of 126 challenges resulted in reversals. During the playoffs, 15 reviews were called for, with six reversals. Reversal rates have increased over time. In 1999, only 29 percent of challenges were successful, compared to 40 percent in 2009 [source: Myers].

The replay assistant in the booth often has to make some of the most crucial review decisions. Read on to find out how those rules work.