Beast Mode 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. She says, "Reverse mode. Secret code: The Beast!" to initiate the process, which jettisons the shoulder pylons, painfully releases the Eva's limiters, and triggers a physical transformation wherein the Eva "loses its humanity". The reconfigured Eva is still controlled by the pilot and relies on internal battery power, but exhibits vastly heightened mobility and A.T. Field-breaching power.
Both the Eva and its pilot behave in a very animalistic manner. Activating Beast Mode seems to be very taxing on the pilot, and entails a negative plug depth that puts her at great risk.
It is unknown if any other Eva units possess the Beast Mode function.
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