Jack explains that his unique magic Unleash Beast (hidden meaning: The howl that breaks the moonlit night) enables him to transform himself into a wolf without the aid of a potion, and in Book 7.5 we learn that it is a “reversion” spell rather than a “shapeshifting” spell.
When Ace tries to practice his magic-swapping UM on Jack he fails to transform into anything, with Jack saying that Ace “just got all hairy like a yeti.”
Later Jack explains that he can only use Unleash Beast because of his wolf ancestry, saying. Riddle asks, “Does that mean it’s a reversion spell rather than a shapeshifting spell?,” which Jack confirms: “So when Ace used it, he didn’t turn into a wolf. He just got all hairy.”
Jack acquired his unique magic the earliest out of the first-year group, but he says there wasn’t any particular impetus: “My maternal grandfather used a reversion signature spell just like me. I really looked up to him. I kept trying to imitate it, and eventually it just clicked.”
On the topic of his unique magic Jack says:
My mind stays the same. Some of my senses get heightened, though. The biggest change is to my sense of smell. It’s way sharper than in my human form. I can identify the game on an entire mountain by smell alone. I can also tell who last passed through an area by the scent in their tracks. So I know when something dangerous is headed my way, like a bear.
Jack says that having such a keen sense of smell also has its problems, however, as strong scents can make him dizzy, such as synthetic fragrances like cologne.
Jack says, “Running feels way better when I’m transformed. I can run 60 to 70 kilometers an hour, so I cover a lot more ground. And at that speed, I can pull off jumps over 10 meters high.”
He also says that being in wolf form constantly drains his magic reserves, so he has to be careful. He says he spent his last holiday at home pulling a sleigh in his wolf form to enhance its strength and duration.
Jack says, “I have to stay focused, or else I can de-shift without warning. Sometimes I have trouble mustering up much strength too.” Deuce says that they learned in class from Crewel that transforming into another creature without the aid of a potion requires extremely high-level magic.
We see Jack use his unique magic in Book 2 and during Vargas Camp.
Jack also uses his unique magic during Book 7 in Leona’s dream.
Twisted Wonderland Reference Note 