A Narrative Device for handling tropes.
If an author has a trope that turns out to be popular, or that he plans to use frequently, he writes a Trope Trigger into the storyline so that whenever he needs the "triggered" trope he just drops in an instance of the Trope Trigger. Depending on the form of the trigger, it might be used for Foreshadowing: when the audience sees the trigger, they know the "triggered" trope can't be far away.
The MacGuffin is nearly always an object, while the Trope Trigger may or may not be. The MacGuffin usually has no function except as the thing that everything revolves around. The Trope Trigger is defined by the trope it activates.
The Plot Sensitive Button is a physical object that is used repeatedly throughout the work but produces different effects at different times. The Trope Trigger always produces the same effect.
If a character with a Berserk Button simply flies into a rage when the "button" is pushed then it is not a "triggered" trope in this sense. Anger per se is not a trope. If the character's Berserk Button always triggers a specific anger trope (such as Agitated Item Stomping or Hulking Out), then the Berserk Button is also a Trope Trigger.
- The resident idiot in The Slayers, Gourry Gabriev, allows Lina Inverse to be Mr. Exposition just by being an average person.
- Full Metal Panic!: Every time that Sousuke brings his military background into the school (such as Stuff Blowing Up), Kaname hits him. This Trope Trigger activates Paper Fan of Doom or Wrestler in All of Us depending on how angry she gets.
- Smallville: Several episodes required Clark Kent to become Brainwashed and Crazy. The writers handled this by retooling red kryptonite from the Superman canon, so that instead of having some bizarre random effect on Clark it always turned him into a sociopath. It got so whenever fans saw a red glowing rock they knew what trope would be in force for the episode.