WARNING: The following article contains spoilers for Episode 76 of My Hero Academia, "Infinite 100%."
Midoriya's power, One For All, is incredibly unstable. The quirk, passed down from hero to hero, grants whoever wields it strength beyond imagination. With it, All Might became the symbol of peace and the number one hero in the world. However, Deku, being a fairly frail boy at the start of the series, couldn't harness all his strength without breaking his body. He therefore resolved to learn how to control his strength, only unleashing small chunks of it per fight.
That is until Episode 76 of the anime, where Midoriya figured out a way to break through the limits of his own power and, in the process, break the laws of time itself.
DEKU AND ERI'S QUIRKS
Returning to the early episodes of My Hero Academia serve as a reminder as to how far Deku has come as a hero. Early on, he regularly unleashed 100% of his strength into single strikes, but, due to how little control he had over his quirk, these attacks were often uncontrollable and left Deku with broken limbs. It took until midway through the second season before Deku could manage to control his quirk without breaking his body, thanks to Gran Torino's training.
By the time Deku stormed Overhaul's base, Deku could control his quirk well enough to deliver blow after blow. Each strike was weaker than his old ones but his skill made him a greater threat, allowing him to strike again and again without breaking in the process.
But as it turns out, Deku isn't the only one with a powerful, uncontrollable quirk. Eri, who we learn this episode is not Overhaul's daughter but, rather, the granddaughter of the yakuza boss who took Overhaul in, has the quirk, Rewind. This quirk allows her to rewind matter backward, causing people who come into contact with her to revert back to primeval matter.
Overhaul sees this as a way to create a war, where he would delete quirks everywhere, then use Eri's quirk to create drugs that restore them. He'd sell quirk enhancements to the villains, then sell quirk revitalization to the heroes, creating a chain of demand with him in the center of it.
However, Eri has no control over this. Overhaul just used her body, knowing how to manipulate Eri's own blood to create the darts. So, Eri bore little love for Overhaul. Wanting to be safe, she dives right into Deku's arms, right as he is charging after Overhaul to beat the villain into submission.
REWINDING THE DAMAGE
With the two united, however, something happens. When Deku unleashes the full force of One For All on Overhaul, it sends the villain flying, but Deku's bones don't break. While he's at first stunned, he soon realizes what's going on.
As soon as his bones broke from the strain of using One For All, Eri's quirk, Rewind, returned his body to its prior state, healing him up before he could feel his injuries. This doesn't allow Deku to necessarily use One For All without repercussion -- it's just these repercussions are balanced out by Eri, who Deku binds to his back to both keep her away from Overhaul and keep her close so she can use her quirk on him.
At this point, Overhaul's quirk allows him to absorb even more of his henchmen, becoming a colossal monster that's literally as large as a building. The henchman he absorbs were able to subdue both Uraraka and Tsuyu earlier in the episode just by being near them and assimilating their life energy. This makes Overhaul stronger than he ever has been, and yet... Deku sends him flying into the stratosphere with one hit.
Needless to say, One For All at 100% is absurdly powerful and, with Eri's quirk, Deku is able to scatter Overhaul's body to the winds. The fight continues in the clouds, since the sheer force of Deku kicking the air sends him launching like a rocket up there. Even with Overhaul's quirk allowing him to instantly reassemble his flesh, he can't regenerate fast enough to keep himself together. Deku beats Overhaul into the pavement.
BREAKING TIME
But there's another catch to all this. In the prior episode, Sir Nighteye saw the future: Deku would be unable to stop Overhaul. His predictions, which are never wrong, foresaw that Deku would be torn asunder by Overhaul. However, as we saw at the end of this episode, Deku wins, remaining mostly unharmed by Overhaul, who Sir Nighteye foresaw would escape.
This means that Deku may have altered the future, which Sir Nighteye up until this point believed impossible. Considering that Sir Nighteye saw All Might dying in the future, this is a huge deal. This means that any of Sir Nighteye's predictions, while should come true, might not necessarily be set in stone if something unforeseeable happens.
How could this happen, then? It's possible that All For One is just so strong it can alter the unalterable flow of time. All For One, after all, is a very powerful Quirk. However, it's more likely that Eri's influence, and not Deku's, contributed to the time alteration.
After all, Eri's restorative quirk is all about breaking the laws of time. With this in mind, it's highly possible that, because of Eri using her quirk on a near-constant basis, that the flow of time Sir Nighteye witnessed and used to make his prediction of the future was interrupted or skewed. After all, in no timeline should Deku heal rapidly after unleashing One For All. In any normal timeline, the moves Deku pulled off would've ripped his body apart.
Regardless, it's clear that nothing can stop One For All at its peak -- not even the inexorable flow of time itself. This means that, potentially, whatever threat promises to snuff out All Might's life in Sir Nighteye's prediction might still be stopped. Deku will just need to hope Eri masters her quirk soon.
My Hero Academia is available for streaming on Crunchyroll, Funimation and Hulu. New episodes are released every Saturday.