Board Thread:Anime/@comment-26131853-20150718221255/@comment-75.176.164.75-20160208155840

Isayama-san explained this, guys. Ackermans (like Mikasa and Levi) possess some kind of special energy that makes them super-good at fighting (Ackerman Energy, I think it's called), but an Ackerman has to go through like, something really traumatic to awaken their powers. Remember that whole thing in episode 6? Yeah, that was her awakening her Ackerman powers. It's not a mystery anymore.