In Evangelion 2.0, the mode is showcased when Mari pilots Unit-02 against the Tenth Angel. After traditional combat methods prove ineffective, she initiates the process by saying, "Invert mode. Secret code: The Beast!"
This jettisons the shoulder pylons, painfully releases the Eva's limiters, and triggers a physical transformation wherein the Eva "abandons its humanity". The Eva's body proportions are generally elongated, and the head becomes saurian in appearance, with a large maw and uniformly shaped teeth. By the end of the transformation, many of the Eva's outfittings are missing or otherwise altered. 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 for losing human form herself. The only unambiguous physical change that Mari experiences as a result of the venture are glowing green irises.
It is unknown if any other Eva units possess the Beast Mode function.
In Evangelion 3.0, Unit-02 used Beast Mode again while being piloted by Asuka. This time, however, the activation code for it was "Code 777". This one is distinctly more beastly in appearance, growing a long tail, the joints morphing into digitigrade ones, and extended fangs from its jaw. It demonstrated the already seen heightened speed and strength, using its jaws against Mark.09 that was taking over the AAA Wunder while under SEELE's control.
The term "Beast Mode" is not used within the film itself, but rapidly entered the fan parlance. (First appeared as a fan coinage?) It appears on the packaging of a set of Eva figurines released by HDM Souzetsu, and possibly other merchandise, which gives "Beast Mode" sufficient authenticity.
The "official" term for the mode, as used by Ritsuko in-film, is Beast Form 2nd Phase (Type 1) (獣化第２形態（第１種）, Juuka Dai 2 Keitai (Dai 1 Shu)). This raises the questions of what "1st Phase" would refer to.
- The limiters which were shown during Evangelion Unit-02 turning into the Beast Mode resembles the control rods in Jet Alone.
- This is not well-explained. The one cut addressing it simply shows the plates on Unit-02's back vanishing, as by "sinking into" a dark matrix from which the limiters then eject.
- Full Color Market. HDM創絶エヴァンゲリヲン新劇場版2 ｓｐ入全9種フルコンプ. Retrieved 2011-02-03.
- Translation from Evangelion 2.0 Complete Records Collection.