Defence Mechanism Superpower

A character has a superpower that only works when they're threatened - they can't initiate it at will, and occasionally they can't stop it either. This kind of situation is a common cue for heretofore unknown powers to develop or for existing ones to undergo a dramatic boost in power, either temporarily or for keeps. A storyline in which the character tries to learn to use their power voluntarily is a likely result.

Can be used in a similar way to Always a Bigger Fish, with a cowering hero looking up to discover their aggressor has mysteriously been blasted into dust, and muttering innocently "Did I do that?". Compare Desperation Attack, a similar trope in video games.

Compare Die or Fly, in which a power is initially discovered in this way, but can be used freely thereafter.

Anime and Manga

 * In Neon Genesis Evangelion, if Shinji is in a particularly grave psychological threat, Eva-01 goes into "berserker mode" where it can basically discard its technological limiters and go Super Robot-slash-Eldritch Abomination. Fitting, with how the series is based on Freud's defence mechanism theories.....
 * Son Gohan from Dragonball starts out as a pampered child who can only access his hidden powers when properly threatened or angered.
 * Tower of God: At first, 25th Baam's powers are completely passive, resulting in magic resistance. And occasionally, for example when he and Michelle Light get curb-stomped by Hwa Ryun, they are a last defense and use Shinsoo to cut up faces. And eyeballs
 * Inuyasha. When Inuyasha feels death staring him in the face, his superhuman ancestry goes into a Super Powered Evil Side, which would eventually consume his soul. His sword has a binding spell on it to prevent this.
 * Happened to Kagome in the first episode, while Mistress Centipede is hauling her through the Bone Eater's Well.
 * Jo Jos Bizarre Adventure has a stand called "Achtung Baby" (yes, the entire series is music references). It activates its power of invisibility when scared. Of course, in a series where Heart is anything and everything, this is incredibly useful.
 * Naruto's Gaara qualifies as he is protected by the sand controlled by his inner demon, Shukaku.
 * In One Piece, The Invincible Pearl could manifest flames when he felt threatened, i.e. whenever anyone managed to put a scratch on him through his supposedly invincible defense. While screaming "DANGER!! DANGER!!"
 * Ranma 1/2, titular character has the Neko-Ken technique. His mind "learned" it as a way to briefly escape his crippling ailurophobia when he's absolutely surrounded by cats and is unable to flee from them, turning him into a cat. It happens more often than you might think. While it makes him immensely powerful, lightning-fast, and savage, he still thinks like a cat while in that state, making it an Useless Useful Spell that anyone with half a mind can thwart and defeat.
 * Pacifica from Scrapped Princess is only seemingly able to use her scream attack when under extreme stress.
 * Emergy Maxfell of Scryed can only conjure his alter when he feels threatened, and it gets more powerful the more hopeless the situation gets.
 * In Soul Eater, Death the Kid has this in the form of the Sanzu Lines. First kicks in when Mosquito tries to kill him, then later to allow him to kill Noah. Channelling his father's power at Shinigami's expense, the inevitable storyline of Kid learning to control the Super Mode at will should be...interesting.

Comic Books

 * One of the Kudzu Plot elements of Countdown was Jimmy Olsen suddenly getting all of his Silver Age powers at the same time thanks to getting the New Gods' souls jammed into him. One of them would activate when he was seriously threatened.
 * Empowered's existing powers get much more effective when Ninjette is busy disproving the Law of Conservation of Ninjitsu, as well as in a couple of other situations of extreme danger.
 * Darwin of X-Men/X-Factor sounds like the most literal example. His superpower is nothing but defense mechanisms, of any imaginable variety, for any and all situations, cranked up to the point of invincibility. It's just too bad he doesn't really have anything for offense. So while he can survive almost anything, he rarely has the ability to actually win a fight. One one notable occasion, his powers decided the best way for him to survive fighting the Hulk was to teleport him to the next state over.
 * However in at least one what-if dealing with Vulcan who was prepared to murder everyone in the X-men to cover up how he'd killed all the X-men trapped on Krakatoa in a Superboy-Prime kind of 'No I didn't mean to!' situation Xavier guided Darwin into taking a more proactive, controlled use of his power as he fed it information on what was happening resulting in Darwin's power forming the ability to destroy the part of Vulcan's brain that accessed his powers rendering him permanently powerless.
 * Before that, there was Lifeguard, a short-term member of the team whose powers were "whatever's necessary to get people out of harm's way." Under fire? Armored skin. Someone's falling from a building? Sprout wings. And so on.
 * It could be said that the Hulk could qualify as having such a power, assuming that whatever's threatening Bruce Banner manages to anger him enough.

Fan Works

 * In Damnatus, Nira rarely uses her Psychic Powers unless she is physically under attack. She has a very good reason for this, however, as using them is a good way to get body snatched by the daemon that is currently on the loose.
 * Ringo in With Strings Attached. He automatically teleports to a safe location when he is badly startled. This can be inconvenient when he ends up somewhere hundreds of miles away from the others.

Film

 * Dark City: Prior to getting an Exposition Beam explaining his powers, John Murdoch could only use them when his survival instincts kicked in.
 * Minya from the Godzilla films can shoot a stream of thermonuclear breath, but only when his tail is stepped on.
 * Teeth is a lovely film about a teen girl with Vagina Dentata. These teeth only chomp down when there's something up in there and she doesn't want it up in there... which happens quite frequently throughout the movie.
 * It doesn't help that in the film's universe, All Men Are Perverts.
 * In The Incredibles, Violet Parr gets a boost to her force field powers (effectively permanent until she dissolves them, probably six feet in diameter) when she and her brother, Dash, are threatened by the villian's mooks. Inverted earlier in the movie: when requested by her mother to.
 * The Hulk in The Avengers takes this a step further, emerging not only when Banner is angry enough, but whenever his life is significantly threatened. As Banner learned the hard way, this includes self-inflicted injury.

Literature

 * In Harry Potter, this is how young wizards first manifest their powers prior to learning to control them. In Harry's case for example, his first memory of using magic is apparating away from some pursuing bullies.
 * Lots of wizard children are lucky enough to manifest by levitating a favorite toy that no one will pick up for them, or something--it's just unluckies like Harry and The Amazing Bouncing Neville who have good stories.
 * The main character in Richard Sabia's short story I Was A Teen-Age Secret Weapon has a psychic power that causes anyone who displays hostility towards him to lose coordination and become accident-prone. He never really becomes aware that he has this power, and only the military scientists who have been studying him seem to know about it.
 * In the Mistborn trilogy, Vin decides that she has this. When losing a fight, she is saved by doing something with her existing magic that nobody knew was even theoretically possible - specifically, she . She later counts on this trope coming into play to save her. In the end,
 * Bink, from the Xanth novels, had one of these; people incorrectly believed him to have no magical talent at all because no one had ever seen it activate.

Live Action TV
"Tenth Doctor: They have survived this long because they have the most perfect defence system ever evolved."
 * Piper from Charmed, who had the power to freeze time, activated by being startled or scared through most of the first season.
 * Likewise for Phoebe's levitation, which activated during a fight with a demon, and Paige could only orb for a long time when startled or in immediate danger. Phoebe also mostly received premonitions to inform her that some innocent's life was in danger.
 * "The Morgan", as demonstrated by Chuck, can cause heart attacks and slippage near very high and broken windows.
 * Now we have Intersect 2, which is essentially a bucket full of one Defence Mechanism Superpower after another.
 * 2.0 actually both subverts it and plays it straight. Chuck can only use it when he's not overly emotional, which means he can use it when he's not in danger. The moment things get stressful he can't use it, until he really needs to, at which point he emotionally either enters the eye of the storm (if somebody else is in danger) or has to use a calming mantra (if he's in danger).
 * Possibly a bit different, but in Doctor Who, the Weeping Angels turn to stone when they are being looked at.


 * When they reappear in "The Time of Angels", it is clarified that they aren't just stone--you cannot, for example, break them with a hammer--but somehow out of phase with the universe in such a way that they cannot be damaged at all in stone mode. The word quantum may be involved.
 * Maya on Heroes, who goes all Everybodys Dead Dave when she gets scared, and can originally only counteract it in the presence of her twin brother.
 * Also from Heroes, Elle surges with thus-far-unseen levels of electricity as a response to Sylar trying to slice her head open.
 * The boy from the pilot episode of Sanctuary.
 * Angel had an episode with a girl whose telekinetic powers were this way. They started as a result of sexual abuse by her father and kicked in when she got threatened.

Tabletop Games

 * GURPS has the limitation "Emergencies Only" to simulate this.
 * This is a major schtick of white cards in Magic: The Gathering. In particular, Righteousness and Smite come to mind.
 * In the Old World of Darkness, this was how a werewolf would first manifest his powers--shifting to Crinos form when threatened and leaving his would-be-tormentors as pink mist. At least one edition of the core book had a foreword that was written as an experienced werewolf talking down one who had changed for the first time, explaining to him that what just happened was okay and (relatively) normal.

Video Games

 * In Golden Sun, adepts have forms of Psyenergy which unconsciously protect them when they need it. This is first demonstrated in a cutscene in the first game when . They vow to make an attempt to master this unconscious power, but never actually learn to willingly trigger it.
 * Mario and Luigi Bowsers Inside Story gives this to Bowser. Whenever he gets himself crushed by something massive, his "Rump Command" becomes active and requires the brothers to pump him full of energy. The end result is that Bowser grows to Godzilla-size, ready to Megaton Punch whatever just smashed him.

Web Comics

 * In Minion, the only time we've ever seen Gin in werewolf form was after a skeleton warrior KO'd him with a morningstar.
 * It's hard to say for sure, but it seems like the Monster in the Darkness, from Order of the Stick, might have some of these - at least whatever it was that happened in #661. In this case, it's not the MitD itself that is threatened - who knows what could do that - but those it has come to care about.

Western Animation

 * Adventures of the Galaxy Rangers. Super Soldier Shane Gooseman of the Galaxy Rangers has the reflexive ability to adapt into a form that can withstand whatever hazardous environment or assault he's being exposed to.
 * Obligatory Avatar: The Last Airbender example: The Avatar State starts out as this, presumably for every avatar. With effort and meditation, though, he or she can go in and out of it at will.
 * It should be noted that until they reach the level where they can control entering/exiting the Avatar State, the Avatar has very little direct control over their actions in the Avatar state at all. Aang unintentionally dished out some epic destruction on Earth Kingdom soldiers when a general provoked him into the transformation in an attempt to help/force him to master the state before he was ready. He also almost certainly killed more Fire Nation soldiers than he would ever be comfortable acknowledging, back when he fused with the Ocean.
 * The title character in The Iron Giant is a massive alien war machine that, due to damage caused during the initial landing on earth, is completely unaware of its true purpose. Unfortunately, the Giant's weapons and more destructive mindset can be triggered against its will if it perceives a threat.
 * The Blowfish Avenger in The Tick.