I like the idea of the Avatar State being an automatic protection device, but it also seemed like A:TLA suggested it could be triggered by being hyper-emotional (specifically, I just re-watched The Ember Island Players and Aang metions being so upset he should be in the Avatar State).
It seems like Korra is pretty emotionally volatile, but hasn’t entered the Avatar State that we know of (but hasn’t really been that close to death either).
Aang was able to Avatar State pretty early on despite not having mastered all four elements OR his chakras. He may not have been able to control it, but he was able to do it. Korra seems incapable.
But, if you had to ask me, the flashbacks were saying SOMETHING about the Avatar State or Eneergy Bending and we only got the surface message.
-Da7e-
