振り切った演技 is used to describe an actor has completely gotten into a character and played the role convincingly. 迫真の演技 is similar, but 振り切った演技 tends to be used for a difficult and eccentric character. Playing a 中二病 boy typically requires 振り切った演技 because, you know, it's a bit embarrassing and eccentric role.
This 振り切る can be understood either as "to shake off your ordinary way of thinking and become unbound by anxiety and embarrassment" or "to fully put your effort into the character". 振る can mean to put/direct/assign (energy, workload, role, number, etc) to something/someone. 切る adds the meaning of "completely". (gamer's example: パラメータをSTRに振り切る "to assign all parameters to strength")