Board Thread:General Discussion/@comment-50.165.73.211-20140815010226/@comment-24435688-20140815042952

it's possible that her semblance is tied to her aura and thus has a limit. A bit like how Weiss was really tired after the Nevermore fight, presumably from using so many glyphs in such a short amount of time; and how Ruby might have tuckered out when she was carrying Penny (though that could have been that Penny was just really heavy.)

So, her limit could be the same limit any container has: it can hold a certain amount of something, but one it reaches its limit it will either break or start to overflow and (in the case of Yang) become susceptible to whatever she was previously absorbing.