- With the exception of the last two minutes in each half and all of overtime, a coach can issue a challenge for a play he feels is unfair. The referee then has 90 seconds to review the play via instant replay and determine if it stands or not. There must be compelling evidence to overturn a call--questionable calls are very rarely overturned. Each coach can make two challenges per game but cannot make a challenge if his team has no timeouts remaining. Should the coach lose the challenge, his team will be charged a timeout. If a coach is successful on two challenges, the team receives a third challenge.
- Not all plays are subject to a challenge. Passing plays, sideline plays, goal-line/end zone plays, field goals and detectable infractions are subject to challenge when necessary. Coaches cannot challenge the status of the clock, downs, force outs, recovery of a loose ball, forward progress or inadvertent whistles. (The list is not limited to such plays and remains at the referee's discretion.) Officials in the replay booth can order a replay in the last two minutes of each half and all of overtime.
- Originally, coaches could signal a challenge by the use of a pager or by throwing a weighted red flag onto the field. The flag was supposed to be a fallback in case the pager didn't work. But a number of coaches--most notably then-Pittsburgh Steelers coach Bill Cowher--preferred using the flag because it left no doubt as to the coach's intention. With game play moving quickly and a challenge impossible after the next play has started, it made more sense to toss a flag onto the field.
