This Quirk allows the user to build up kinetic energy by repeating regular motions and store it for later use. When the user chooses to release the energy, they gain an explosive burst of speed and power.
The user can choose to only use some of the stored up energy at a time, allowing them to achieve multiple bursts of speed and power.
The addition of the already stockpiled energy that makes up One For All greatly increases the power of Fa Jin, to the point that just 45% of One For All's power, in-conjunction with energy stored up by Fa Jin and slinging with Blackwhip, is enough to reach comparable 100% levels, and without the backlash to the body.
Deku uses this Quirk for the first time while fighting Lady Nagant. He uses Smokescreen to hinder her vision so he can jump around from building to building, gathering kinetic energy before jumping out of a skyscraper and grabbing her. He then proceeds to re-store up more energy, demonstrating the power of the Quirk with One For All, first to become faster than a bullet to rescue Kai Chisaki, and then to execute a Faux 100 Percent: Manchester Smash to finally defeat Lady Nagant.
- Fa Jin is one of the oldest known Quirks in the series along with One For All, All For One, Life Force, Spearlike Bones, and the unnamed luminescence Quirk.
- Fa jin - Wikipedia article about the Chinese martial art technique this Quirk is named after.