Ace’s unique magic Joker Snatch (hidden meaning: Gimme Your Trump Card) switches magic and/or a spell (EN uses both words) between Ace and a target of his choosing, manifesting during the climax of Book 7 when he uses it for the first time to swap Riddle’s unique magic with a spell/magic of his own and then uses Riddle’s “Off with Your Head” on Riddle himself.
In Book 7 the first-year students help Ace practice Joker Snatch, with Ortho observing, “Sebek and Ace have switched spellscript structures!” when he uses it on Sebek to receive Sebek’s Living Bolt.
When Ace electrifies himself and moves “barely 30 centimeters” he says, “You’re telling me this spell comes with its user’s glaring inexperience?!,” but Sebek insists that the inexperience lies with Ace, not himself, so the limitation of “Ace can only use a UM to the extent that it can be used by its original owner” is maybe still unproven.
Ace says, “Sebek’s unique magic shifts his body into a completely different form of matter. For spells like that, even if I understood how they worked, my body couldn’t keep up,” so Sebek’s UM might be of a type that is very difficult or impossible for Ace to utilize.
Jack says that when Ace tried to use his Unleash Beast he did not transform into any animal, but instead “just got all hairy like a yeti.”
This might, however, have not been a result of Ace’s inexperience: Jack’s UM is not a shapeshifting spell, it is a reversion spell, only enabling him to transform into a wolf because he is already a wolf beastperson (more here ▶︎).
Epel says that when Ace tried to use his Sleep Kiss he trapped Deuce by mistake rather than Epel himself.
Ace explains:
(Joker Snatch) just switches one spell with another. I’m not “robbing” people! It exchanges a spell of mine with a spell from the target. That’s it.
I also can’t switch spells when I don’t know what kind of spells someone uses. If I’m expecting a fire spell but then it turns out to be a lightning spell, it doesn’t work.
Plus, the target has to have fully formulated their spell, or mine will fizzle out.
Ortho says, “you have to have a read on the cards in the opponent’s hand to accurately make the switch,” and Cater agrees, “So you have to anticipate what your opponent will play next, ahead of time…like a card game.”
When Ace wonders why he had been able to successfully trade for Riddle’s spell in Book 7 Ortho observes:
The more data you have, the higher your odds of success will be. That data includes things like the timing of when the target spell is cast, as well as its effect and potency. In your case, Ace,
You’ve had Off With Your Head cast on you so many times it’s become commonplace.
Maybe that’s why you succeeded at making the switch, despite having only a moment to act.
Twisted Wonderland Reference Note 