All Might, Izuku and Katsuki try to determine how to activate the Blackwhip quirk. He can’t seem to figure out what activated it before. Katsuki seems to believed that it requires Izuku to be in danger. Finally, in on the secrets, Katsuki asks what All Might knew about the Blackwhip quirk. He admits that both he and his master were ignorant of the fact that “One For All is capable of letting Izuku use previous wielders Quirk.”
Throughout this series it felt like Izuku was going to become exactly like All Might. Following in his footsteps as All Might is his idol. Slowly, Izuku is separating himself from his idol. Now with this new situation Izuku is coming into his own. Now traveling a path that even All Might didn’t, Izuku’s story becomes more interesting. In order to get Izuku to release his Blackwhip ability he may need to find himself in danger once again.
Eri, Izuku, Mirio and Neito gather outside the dormitories. Neito attempts to copy Eri’s Rewind Quirk. All he does is grow a horn. The horn itself is necessary to use the Quirk, but he can’t. He explains that ” as Rewind is an accumulation-type Quirk like One For All, it isn’t possible for him to use it.”
He also states “that his Quirk allows him to copy the fundamental abilities of a Quirk, not something requiring stored-up energy as he cannot copy what has already been stored. Izuku is relieved that he has this mechanism as it prevented him from being damaged by One For All when he copied during the Joint Training Battle.” Shota explains that he wanted to see if by Neito copying Eri’s quirk, she could learn to master it herself. Considering her age I think Eri will learn how to use her ability as she gets older. It’s clear that “accumulation-type” emitter quirks are the superior type of quirks.