One For All

One For All (ワン・フォー・オール) is a Quirk used by All Might and Izuku Midoriya.

Information
Little is known about this ability and how exactly it works, however it has been shown to grant the user momentary superhuman strength at destructive levels, increased agility and superhuman speed. The Quirk receives its name from the fact that it can be "inherited" by others. The method of inheritance is not clear but All Might has shown to give it to others by having them ingest a strand of his hair. Proper usage of the Quirk requires intense and rigorous training of the body to prevent the successor from being torn apart from the raw power. Even with the training, the Quirk still puts intense stress on the users body (as when Midoriya first used it he broke his arm).

When Izuku was about to lose his match against Hitoshi, because of his Quirk, he felt the presence of multiple people coursing through his body and activated One For All to break free from his control. Izuku theorizes that since his Quirk is the "sacred torch that is passed down through the ages", those people that he felt were the lingering presence of those who had this power before him. He also mentioned seeing someone with the same hair style of All Might among those who appeared in front of him indicating that his presence were imprint on the Quirk.

Application
According to All Might, to activate One For All, the user must scream: "SMASH!"

Users
All Might: As a well renowned hero, All Might is a proficient user of this Quirk. He is capable of controlling his own strength and focusing his power onto any extremity he chooses without recoil. Due to an untreatable wound that has been inflicted on him, he can only utilize his power for three hour straight; any more would strain his body. And if force to use his Quirk more than he should, he may shorten his time limit for using his Quirk as a repercussion. Izuku Midoriya: Izuku is the new successor of "One for All" chosen by All Might. He has displayed incredible bouts of strength, however, because of his inexperience in regulating his power, it usually results in the destruction of parts of his body. Although, he is able to moderately distribute the power to specific limbs such as his fingers without causing major damage to himself. When first using the power against a living being and knowing it could kill someone, he was able to properly manage to use the Quirk with much more efficiency and preventing the usual destruction of his body from happening, although he can only used it subconsciously as of now. Izuku's control of One For All and the power he can gain from it is 5 out of 100 according to All Might. Using that small percentage of One For All, Izuku can create wind pressure powerful enough to shatter Shouto Todoroki's ice (albeit at the cost of breaking bones).

Trivia

 * To date, all of the moves using One for All are named after states in the USA (e.g. Missouri Smash, Delaware Smash). This is consistent with the manga artist's desire to model All Might after American comic book heroes, especially in All Might's hero form.