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For other uses, see Trigger (disambiguation).
A Game Trigger is a game design element that allows actions in a game to initiate some kind of event. Typically, this happens when a "switch", literal or figurative, is activated after a certain specific objective has been completed or a certain criteria is met.
Campaign[]
Triggers are extremely common in the Call of Duty game as they are an important element in creating the franchises cinematic, blockbuster presentation. Most of the Campaigns are populated with triggers for Game Events. Game events, in this sense, often occur after completing an objective and command teammates and even enemies to do something else, such as move up to the next area or take cover behind a specific object.
In Call of Duty 4: Modern Warfare, and Call of Duty: Modern Warfare 2, a common usage of a trigger is in the playable character's squad. More often than not, the players squadmates will not move up to the next area unless the player moves past a certain point with them.
Disrupting a trigger can lead to an inexplicable death, such as at the end of Heart of the Reich in Call of Duty: World at War, if the player tries to kill the German soldier that burns Chernov with an M2 Flamethrower, they will be immediately killed, as the game needs the event to take place in order to progress. By contrast, being able to choose to save Polonsky or Roebuck represents a trigger that can be manipulated by the player organically.
Zombies[]
Triggers are particularly key to the design of Zombies Mode of the various Call of Duty games, particularly earlier entries where obscure actions would progress Easter Eggs, including the maps main quest. Additionally, the Power Switch present in most maps is a great representative of a literal, player activated trigger, changing the map to open new zones and allow access to new utilities.
Source: https://callofduty.fandom.com/wiki/Trigger. This is an independent Call of Duty: Modern Warfare 4 reference page.