WARNING: The following contains spoilers for My Hero Academia Chapter 313, "High-Speed, Long-Range Mobile Cannon,” by Kohei Horikoshi, Caleb Cook and John Hunt, available now in English from Viz Media.
Alongside One For All’s super strength, Deku has steadily been unlocking the other Quirks of its previous holders. In My Hero Academia Chapter #313, he debuts a new ability, Fa Jin. He hasn’t shown off its full potential yet but from what the manga has revealed so far, Fa Jin promises to become a tool powerful -- enough to bring Deku closer to his goal of becoming the #1 hero.
Fa Jin, Deku's Most Explosive Quirk Yet
Out of the five Quirks Deku has unlocked so far, Fa Jin is the only one with overt offense capabilities. Blackwhip, Smokescreen, Danger Sense and Float have all been primarily support-oriented, providing increased mobility options and perfectly augmenting One For All’s primary power-stocking ability. Fa Jin, on the other hand, might help Deku take the fight to his opponents, and in explosive fashion, no less.
Originally belonging to One For All’s third user -- who is as of yet unnamed -- Fa Jin allows its holder to build up kinetic energy within their body and release it all from any part they wish in an instant. When used, it dramatically increases Deku’s speed and power. The energy’s sudden release suggests that it might be difficult to control at first, but constant practice (as Deku is wont to do) is sure to make the Quirk more versatile than its current state.
Fa Jin, even in its unpolished state, has already proven invaluable in battle. Using this Quirk for the first time, Deku is able to get the drop on expert marksman, Lady Nagant in the latest chapter. Being a ranged fighter, her primary attack strategy when confronting Deku is to maintain her distance and pick him off with her rifle. Her lightning-sharp reflexes and newly acquired Air Walk Quirk made a slow, systematic approach impossible, prompting Deku to adopt a more direct plan.
How Fa Jin Works
Using Fa Jin, Deku is able to leap straight through every level of a multi-story building and he still has enough momentum left to catch up to the airborne Lady Nagant. It’s obvious that the raw power Fa Jin gives him is off the scale -- but how exactly does it work?
The real-world martial arts technique of the same name allows its users to produce high levels of power with relatively little force. Popular moves such as the One Inch Punch are derivatives of Fa Jin. If the new Quirk works similarly to this technique, it opens up a whole new world of One for All possibilities for Deku.
Deku’s primary obstacle in his quest to draw out One for All’s full potential has been his body’s inability to keep up with the upper limits of its power. Fa Jin might offer a workaround until he can safely use "100%." Activating this Quirk while remaining within his allowable limit would increase the power behind his blows without any risk to him. As he grows and becomes more accustomed to One for All, Fa Jin would still continue to supplement his strength. It’s possible that Deku at 100% would be able to produce several times more power than All Might at his peak, due to this new Quirk.
Fa Jin wouldn’t apply to only Deku’s smash attacks, either: It could also boost other moves he’s developed such as Shoot Style and Air Force. Shoot Style already enables Deku to travel faster than most people can perceive so a Fa Jin boost could eventually give his movements the appearance of teleportation. Deku’s still quite a way off from mastering this new power in the My Hero Academia manga though, so it might be some time before its full potential is realized.