Shinji pulling the grip trigger UP (with little force).
Soluzar: This, I never understood. It's a gun. She's holding it in her hands. Hence, Shinji surely should just think 'pull trigger' and presto! Why the need for mechanical triggers?
Reichu: I guess this relates back to Shin-seiki's comment in episode #02, C-192B. At least pulling on a trigger is one of the few actions these pilots can do seated that actually corresponds to what the Eva does.
Soluzar: So perhaps the triggers in the cockpit are just dummies, to make it easier for the pilots to control the real Eva's hands? That sounds very plausible, now you mention it.
Hexon.Arq: If the arms were bound strictly to these "butterfly controls", I could just picture Eva only being able to do some kind of funky "pogo stick" dance. I suppose attaching control gloves to the seat would have been too much work to utilize in every entry plug scene. That would have been interesting, Shinji's fingers spasming in the control gloves as the entrails of Eva… well, you'll see.
OMF: Maybe the triggers are just a failsafe, to ensure that megatonnes of firepower doesn't accidentally go off as the pilot is thinking about their next move.
Dr. Nick: (Megatonnes = exaggeration to the max.)
Reichu: I'm not so sure about that. EVA-02 depletes all of the positron rifle's ammo in #22, and without Asuka even touching the triggers. (But then, if there's any substance to my pet theory about that sequence…)