Beast Mode (ビーストモード) is a special built-in combat function that gives an Eva pilot access to more of an Eva’s innate power, resembling a “controlled berserk”.
In Evangelion 2.0, the mode is showcased when Mari pilots Eva-02 against the Tenth Angel. After traditional combat methods prove ineffective, she initiates the process by saying, “Invert mode. Secret code: The Beast!”
Source: Wallpaper Abyss