It places the arm up to where its missing left arm was.
Mbryo: This is the first time that we see Unit 01's teeth exposed. I'm guessing that this is supposed to be some kind of a reveal, showing the eerie resemblance between an Eva's teeth and human teeth. The red hexagonal things seen in her mouth earlier in the series are merely restraints intended to keep her mouth forcibly shut. The problem is that Unit 01's real teeth seem to come out of nowhere in this episode; they were conspicuously absent when she opened her mouth in the previous episodes. And from this point onwards, the red 'fake teeth' are completely replaced by real teeth, and are only briefly seen in a single scene in EOE where an animator decided to draw them for some reason.
