By your logic, Hagoromo's sword is an asspull, Susano'o is an asspull, Rinne Tensei is an asspull, Reaper Death Seal is an Asspull, and Mangekyo Sharingan is an asspull
I understand what you're getting at, somebody using a jutsu we've never seen before doesn't necessarily make it an asspull, really it's just raising the stakes. However...
"An Ass Pull is a moment when the writers pull something out of thin air in a less-than-graceful narrative development, violating the Law of Conservation of Detail by dropping a plot-critical detail in the middle, or near the end of their narrative without Foreshadowing or dropping a Chekhov's Gun earlier on.
In cases where a character suddenly gets a new skill without explanation, it's usually explained away as a Chekhov's Classroom or Chekhov's Skill, except the audience never saw the character attending the lecture in question, or any prior examples of him or her using, or even training that skill.
An Ass Pull used to resolve an unwinnable situation for the protagonists is a Deus ex Machina. An Ass Pull used in the same way for the villains is a Diabolus ex Machina. An Ass Pull doesn't necessarily have to resolve or derail a situation, though; many times, an Ass Pull is just used without any greater plot implication. Please limit examples on this page to ones that don't fit in either of the other two.
The term is also used to describe something that the characters make up on the spot. See also Shocking Swerve and Writing by the Seat of Your Pants. Given certain examples, Wing Pull could fit into this as well. Sometimes called Sulu's Foil, since it's the opposite of Chekhov's Gun. In Russia, it's called "grand piano in the bushes". Pulling a useful object out of seemingly nowhere is related to Hammerspace.
As this trope frequently concerns plot twists, there are unmarked spoilers below. "
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