Well maybe I can answer your question somewhat. Aang and Azula were traveling down the chute things pretty fast, and when Aang shot his air blast the opposite way he was traveling, it's overall speed wasn't that high especially in relation to how fast they were already moving through the surrounding air. So when Azula positioned her hands in front of her in a wedge shape, it probably wasn't too hard to slice through a weakened air blast. Perhaps the only thing this would have done (as far as I can figure according to the real world laws of physics anyway) would be to speed Aang up a little, and slow Azula down when she hit the blast. And since Aang was trying to get away from Azula, that may have been a primary reason Aang shot his air blast at all. Just my two cents.