Board Thread:General Discussion/@comment-67.70.92.41-20151228204640/@comment-4010415-20160226221154

Yeah, they likely didn't have the "Aura depleted" effect yet, especially since they hadn't even decided for certain what kinds of things Dust would do.

In the manga, Weiss had already been training before the Giant Armor came out, so for all we know, that aspect of the manga could be canon. In which case, Weiss was already operating on less than 100% Aura when she fought the Giant Armor, and it punched her so hard that it overpowered her Aura.

The handy thing about Myrtenaster is that, it seems one way Weiss uses Dust with it is that she coats the blade in the powdered Dust and then swings the rapier in order to disturb the Dust, thus triggering it. No Aura required. Just like how Ruby sneezing on powdered Dust triggered it.