One For All: Full Cowl - 100% (ワン・フォー・オール フルカウル 100
During the final battle against Overhaul, Izuku unknowingly went over his personal limit because Eri's Quirk undid the damage to his body before he ever felt any pain. This allowed Izuku to get a feel for how quickly his body gets rewound by Eri.
Using Eri's power to negate the backlash from overusing One For All, Izuku can draw out the maximum power his Quirk has to offer.
Full Cowl at 100% dramatically increases all of Izuku's abilities, granting him strength and speed that rival that of All Might, and the ability to move through mid-air using wind pressure.
At 100%, Izuku completely outmatched Overhaul in his final form. He was able to launch Overhaul's massive form into the sky with a single kick before beating him down with continuous punches that prevented the villain from healing himself.
In this form, Izuku's hair stands up, gaining a spiky appearance, while glowing a brighter shade of green than normal, and yellow (blue in the anime) energy flows from his eyes with his electricity becoming blue instead of the usual green. His skin also adopts a reddish hue. In both the movies and the manga, his pupils and irises become distorted.
When Katsuki Bakugo temporarily received One For All, he gained the same level of physically altered feats as Izuku, albeit with a few differences: his hair became spikier, while glowing a brighter shade of yellow, and the emitted electricity was orange. While using One For All at 100%, Katsuki not only gained similarly enhanced physical strength and speed as Izuku, but his Explosion Quirk became potent enough to instantly melt through solid rock, in a manner similar to lava, as he came into close proximity of it. Using this technique along with Izuku, he was able to defeat Nine in his powered-up state, who ended up being completely incapacitated after the fight.
During Izuku's battle against Flect Turn, he once again unleashed the power of Full Cowl 100% to execute United States of World Smash, demonstrating his ability to move faster than the eye can see, pounding Flect with a barrage of punches before moving around him to build up air pressure and finally knocking him out with a powerful kick. This technique was able to overload Flect's seemingly unstoppable Reflect Quirk.
When Izuku first used this technique, the major drawback was that Eri was still unable to control her Rewind Quirk, and therefore, her ability to maintain Izuku's form became unstable. It is likely Izuku could have been killed had it not been for the intervention of Eraser Head. During the final battle with Nine, Izuku and Katsuki were able to access this technique unaided, despite Izuku only being able to normally handle 10-15% of One For All's power at the time. Their bodies were not blown apart, however they still received major damage to their arms and legs during the battle; this resistance to the blowback may have been due to their exposure to Katsuma Shimano's Cell Activation Quirk, which was stated to boost the body's physical condition. Izuku once again faced heavy damage to his arms after using the technique against Flect Turn, however in comparison to the fight against Nine, Izuku activated it for a much shorter amount of time, and his body had reached 30-45% capability of One For All.