Rewind gives Eri the ability to reverse a living being's body back to a previous state.
When Rewind is activated, Eri's horn grows and glows brightly. Those that are in contact or in the vicinity of Eri at the moment of the activation start having their bodies rewound to previous physical states, undoing or bringing back injuries or modifications from the present or the past. She is capable of reverting people back to a point before they even existed, effectively erasing them from existence. Kai Chisaki even stated that this Quirk can rewind genetics, dubbing it a terrifying power.
During the climax of the Shie Hassaikai Raid, it is shown that Eri's strong emotions play a role in her Quirk's efficacy. Recalling how Mirio Togata risked everything to save her, a hope-infused Eri obtained the determination to help others, causing her Quirk to spontaneously activate, and immediately defuse Kai who had combined himself with Shin Nemoto. And as Izuku Midoriya protected Eri, her Rewind remained active and continued to grow stronger, on which Izuku capitalized in order to activate One For All: Full Cowl - 100%. After the battle, Eri's Quirk began to run awry, had it not been for Eraser Head using his Erasure Quirk to neutralize her power, and eventually, her horn shrank back to its minuscule size.
Because of her young age and inexperience, as well as the lack of information about Rewind, Eri is unable to properly control her ability: indeed, the first instance of Rewind manifesting was Eri accidentally erasing her father. As it only affects living beings, it is difficult to train Rewind safely. These risks have been mitigated by placing Eri in the care of Shota Aizawa, whose Erasure Quirk can neutralize Rewind should it go out of control.
However, by training her Quirk through restoring the limbs of bugs and lizards, Eri has gained some control over her power, being able to consciously activate her Quirk and not let it run awry. This degree of control was enough to restore the Quirk of Mirio Togata, whose ability was erased by a Quirk-destroying bullet during the Shie Hassaikai Raid.
Rewind, as confirmed by Neito Monoma, requires the accumulation of a yet unknown element in order to be activated, otherwise, the Quirk will remain dormant, with Eri's horn remaining small and stubby.
Through several tests and experiments, the Shie Hassaikai was able to develop a drug that could target a person's Quirk Factor, rewinding it to a Quirkless state. The drug consists mostly of Eri's blood, cells, and flesh, which contain the properties of her Quirk. The organization created an antidote capable of restoring a destroyed Quirk using similar methods, though the antidote was never recovered.
After settling in at U.A. High School, Eri began training her Quirk by using it to restore the limbs of bugs and lizards. Through these months of training, Eri managed to obtain enough control over Rewind to restore Mirio's Quirk, Permeation.