A power play in ice hockey occurs after one or more players has violated one of the rules. Once the violation has occurred, the referee will signal that a penalty has been committed, and then will proceed to explain what type of penalty he has witnessed and the jersey number of the skater who will receive the penalty.
Once the referee has signaled the penalty, the player who has committed the penalty must sit in their team's designated penalty box for the specified amount of playing time. Most penalties are typically around two minutes long, and it follows along with the timing of the game itself. This means if the game clock stops, the amount of time remaining in the penalty will also pause. During this allotted amount of time only four players from the same team as the player who received the penalty can continue playing for that amount of time.
Because the team can only have four players, it means that they will have one less player allowed on the ice compared to their opponent. Because there are normally five players allowed on the ice at one time, the team with only four players will therefore be at a disadvantage. Hockey power plays are also considered 4 v. 5 because it describes the number of players on the ice at the time.
During a power play in ice hockey, the team with the advantage, or power play, will remain competing with five players on the ice. The opposing team, or team at the disadvantage at the time will only be allowed to have a max of four players on the ice. This defending team at the disadvantage will typically refer to this 5 v 4 situation as a "penalty kill."
Typically, ice hockey coaches and teams practice potential power plays just the way they would in a game. This means that different teams will be created using players on the same team. The majority of the time coaches will create "special teams" which will be called upon when a powerplay occurs in a real game. This means that they will choose a group of five offensive players to "scrimmage" or practice playing against another group of four designated defensive players.
A penalty kill refers to the exact same situation as a power play, however from the perspective of the other team. The team that is on the penalty kill, is the team whose player received the penalty. This means that only four players from the team can play on the ice at one time, rather than the typical five players. Like with a power play, a penalty kill typically lasts around two minutes, depending on how long the penalty lasts.