system
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human
[A:TLA] Was Aang's victory over Ozai brought about by sheer happenstance?

This has always bothered me. 

It is pretty clear that without the Avatar state, Aang would have perished when Ozai smashed his stone barrier. Instead, of course, Aang was smashed against a stone that just so happened to release his blocked Chakra; Avatar State resumed, righteous can of elemental whoopass opened on Ozai, etc. 

This all makes for a very satisfying turnaround sequence, but it also means that the fate of Aang and likely the entire world apparently depended entirely on that rock being right there. In the world of bending, it seems very little is actually left to chance, which makes this all the more... unexpected, somehow. 

Is it possible that it was Aang's expectation of imminent death, combined with the unexpected pain of his old injury, that re-unlocked the Avatar state? If he hadn't hit that rock just then, would he have still managed to reconnect before his death (apparently seconds away) at Ozai's hand? 

Perhaps some wayward spirit with a gift for foresight just happened to kick that stone into that rock column when it was forming, back in the beginning of the world? I desperately want it to be almost ANYTHING other than chance!
gpt
I think you had it right when you said it was his expectation of death, mortal terror at said death and then the pain that re-focused his mind and allowed the chakra to open, not the physical impact itself fixing his back a'la Dark Knight Rises. 

His spirit would later tell Korra "When we hit our lowest point, we are open to the greatest change", a bit of wisdom that was probably taught to him at that exact moment, though it would take a few decades before he could put it in so many words.