User blog comment:Waters Knight/Weiss killed 'herself' in the White Trailer/@comment-9090085-20131219002528/@comment-9090085-20131219061005

The answer is simple: it's nether - it's shielding.

If you're familiar with Starcraft, for instance, you are probably familiar with how the Protoss faction has energy shields for all their units, protecting their HP from damage until depleted; just like Aura does(the shields also regenerate over time). The thing is, Aura can also be used to heal (among other things), making it a bit more versatile.

On the other hand, if you're Aura is depleted(say from being hit a lot), you can't heal. Hence why they stop duels before someone's Aura runs out, so no one gets hurt.

And judging by how was impressed by Jaune healing a scratch in a few seconds, we can assume this healing depletes Aura rather quickly; limiting what can be healed. (that's also why we're quite certain it's Aura, Pyrrha said Jaune had "a lot of it[Aura]" in reference to that healing)