The old face-like structure being pushed to the side as a new face emerges.
Keisuke-kun: I never understood the purpose of the extra face. The old one is just fine; why does he need a new one?
Reichu: That has puzzled me, as well… Perhaps due to the fact that his face has an ossified surface (which becomes more obvious in the next episode), he cannot heal it normally. When it becomes damaged, he has to shed it completely, and a fresh one emerges from underneath. Maybe the old face would eventually drop off and the new one's ‘beak’ would grow over time to be as long as the old's. But that's just pointless speculation.
felineki: I had never found a reason for the two-head deal, but Reichu's idea seems to work.
Reichu: Of course it does.;;> Another interesting thing you can see in this shot is that Sachiel's “neck” osteoderms and “exo-ribs” are asymmetrical. Although three on each side, the osteoderms are not the same shapes on one side as they are on the other. And the right set of “exo-ribs” does not have a spike on the top, which production drawings suggest is not due to the mine (although animation inconsistencies might lead you to think otherwise).
Also, Sachiel seems to be bleeding a pinkish fluid here as his new face emerges — which is entirely inconsistent with what he'll be bleeding next episode.
Additional Commentary
Hexon.Arq: Perhaps the pink stuff is pus.
Reichu: Perhaps... As we'll see later, another of Sachiel's body fluids is not the color one would expect.
Kendrix: I always found it fascinating how the angels and Evas actually bleed, and quite a lot. It gives the battles a whole different level of intensity, less "cool robot thing" and more as if it were two characters with swords cutting each other to bits.
