Board Thread:General Discussion/@comment-36186343-20200309084119/@comment-2182964-20200309215036

Going by UNMASKED, Segunda Etapa is a form unique to Ulquiorra. Resurrección returns an Arrancar to their original Hollow form, whilst Segunda Etapa returns Ulquiorra to the original form of his tribe. Ulquiorra thought of it as the source of his emptiness and, for these reasons, didn't show it to Aizen. The above is explained in UNMASKED and the short story not be, but be.