"Power play" is a sporting term used to describe a period of play where one team has a numerical advantage in players, usually due to a rule violation by the opposing team.
Temporary numerical advantage in players during a team sport
In several team sports, situations arise where following a rules infraction, one team is penalized by having the number of players on the field of play temporarily reduced. The term power play is commonly applied to the state of advantage the unpenalized team enjoys during this time. Specialized tactics and strategies can apply while a team is on the power play.
Ice hockey
In ice hockey, a team is considered to be on a power play when at least one opposing player is serving a penalty, and the team has a numerical advantage on the ice (whenever both teams have the same number of players on the ice, there is no power play). Up to two players per side may serve in the penalty box without substitutions being permitted, giving a team up to a possible 5-on-3 power play.
There are three types of penalties that can result in a power play for the non-offending team: minor (two minutes), double-minor (four minutes), and major (five minutes). For such penalties, the offending player is ruled off the ice and no substitute for the penalized player is permitted. If a goaltender commits either a minor, a double-minor, or a major penalty, another player who was on the ice at the time of the penalty would serve the penalty instead. A power play resulting from a minor penalty ends if the team with more players on the ice scores. A double-minor penalty is treated as if the player has committed two minor penalties back to back: a goal scored by the team with advantage in the first two minutes only ends the first minor penalty (and the second will start after the game restarts); a goal by the team with advantage in the last two minutes of the penalty will end the power play even if a goal was scored during the first part of the double-minor penalty. If a player is given a major penalty, a power play occurs, but the power play does not terminate even if the team on the power play scores (except in overtime as this ends the game); a major penalty only ends when five minutes have elapsed or the game has ended. A match penalty results in the offending player being ejected from the game (and the player is subject to possible further suspensions), but is otherwise treated the same as a major penalty.
If a team is still on a power play at the end of a regulation period, or at the end of a playoff overtime period, the power play will continue into the following period. "Misconduct" penalties (10 minutes in duration), and "game misconduct" penalties (offending player is ejected for the balance of the game) allow for substitution of the offending player, so do not result in power plays. However, in practice misconduct and game misconduct penalties are often assessed in addition to a major or minor penalty.
Special rules govern situations where three or more players on the same team must serve penalties simultaneously. They are designed to ensure a team can always have at least three skaters on the ice whilst also ensuring all penalties are fully "served" (assuming the game does not end first). In the simplest example, if Andy is assessed a minor penalty, followed by Barry, and both are still in the penalty box when Charlie also receives a minor penalty: Charlie may be substituted prior to play resuming; The two minutes Charlie must serve do not commence until Andy's penalty expires or team on the power play scores, whichever comes first; If the team on the power play scores prior to Andy's penalty expiring but before Barry's, Andy's penalty ends and he may leave the penalty box, but his team must still play two men short until Barry's penalty expires; If the team on the power play does not score prior to Andy's penalty expiring, Andy must remain in the penalty box until Barry's penalty expires or there is a stoppage in play, whichever comes first; Provided there is no stoppage in play, Andy will leave the penalty box when Barry's penalty expires, Barry will leave the penalty box when Charlie's penalty expires (putting their team back at full strength) and Charlie will remain in the penalty box until the next stoppage in play.