Fa Jin

Fa Jin was the Quirk used by the Third User of One For All and later inherited by Izuku Midoriya.

Description
Fa Jin allows the user to build up kinetic energy by performing repetitive movements 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 use only some of the stored 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.

Weaknesses
Not much of Fa Jin is known, but it is suggested that the Quirk takes a long time to build up the kinetic energy, and if released prematurely, then the efficiency of power output may be significantly lower than expected.

Usage
Izuku uses this Quirk for the first time while fighting Lady Nagant. He uses Smokescreen to hinder her vision so he can jump from building to building, gathering kinetic energy before jumping out of a skyscraper and grabbing her. He then proceeds to store up even more energy, demonstrating the Quirk's synergy with One For All, to become faster than a bullet to rescue Kai Chisaki, and then uses it to execute a Faux 100 Percent: Manchester Smash to finally defeat Lady Nagant.

He would later combine Fa Jin with Float and his Air Force technique to allow him to fly across the ocean.

Trivia

 * Fa Jin is one of the oldest known Quirks in the series along with One For All, All For One, Life Force, Spearlike Bones, Gearshift, and the unnamed Luminescence Quirk.
 * Kohei Horikoshi initially wavered between choosing this or "Charge-Up" for the Quirk's name.

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Fa Jin