Traumatic Toggle

If you received a super power or Trauma-Induced Amnesia—or changed bodies with someone else—due to a Freak Lab Accident, lightning bolt, blow to the head, or other traumatic event, experiencing the same trauma a second time will take it away or undo it, just like flipping a switch. Note that for this trope it has to be the exact same trauma reinflicted—if there's any kind of polarity reversal involved with the second exposure, that's Now Do It Again Backwards.

This is of course utter nonsense. Brain damage, for instance, does not appear on odd-numbered blows and disappear with even-numbered ones, it just gets worse. But that doesn't stop some hack writers from using a frying pan as a cast-iron light switch for amnesia or Personality Swaps.

More and more frequently, plots play on the automatic expectation that a Traumatic Toggle will always work, especially when that belief is held by the other characters in a story.

A subtrope of the Reset Button. Compare Now Do It Again Backwards. See also Easy Amnesia.

Examples of Traumatic Toggle include:

Anime and Manga

In the "Am I Pretty?" storyline of Ranma ½, Ranma undergoes a personality change after a blow from Akane causes him to hit his head on a rock. The rest of the cast automatically assumes a second head blow will undo the personality shift, and much of the comedy of the plot comes from repeated attempts to assault Ranma. None succeed, and it takes another accidental head blow to undo the change.

Film - Live-Action

In the 1982 Scott Baio vehicle Zapped!, Baio's character gains Telekinesis from a lab accident; at the end of the film he suffers another similar accident, which apparently removes the powers—or so everyone assumes.
Happened in Déjà vu (1989) with a hitman... who accidentally had his head bumped and forgot everything, so when he was reminded of his cover identity, he thought he really is a "professor of entomology in transit to Sumatra, to catch butterflies". And then hit his head again. And again. Eventually, when he was caught while not knowing what he is doing there, and the mark's thug tried to beat answers out of him, John starts to remember something again:

Johnny Pollack: Punch me one more time.

Subverted in the Mel Gibson movie What Women Want. After he tries to reenact the freak accident that gave him mind reading powers, nothing happens.

Literature

The titular candies in the children's book The Incredible Reversing Peppermints; eating one completely reverses your personality, eating another reverses the reversal. Which does admittedly make a certain amount of sense, at least compared to a few other examples on this page.
It can be said that Faulkner did this in As I Lay Dying. When Dewy Dell (not the brightest bulb) tries to get an abortion, a man tells her that 'more of the same' will get rid of the baby. Dewy Dell storms out of the shop a few moments later raving how it wouldn't work.
In Rosetta's Dress Mess, Rosetta loses her fashion sense after being hit on the head by a plank of wood. Tink surmises that hitting her again will bring it back, and spends the rest of the book trying to whack her without being noticed (as Rosetta couldn't help ducking when she knew the blow was coming). She finally succeeds...after Rosetta got back her fashion sense on her own.

Western Animation

Star Trek: The Animated Series episode "The Practical Joker": After the Enterprise passes through an energy field, the ship's computer starts pulling jokes on the crew. The computer is returned to normal when the Enterprise passes through the cloud again.