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Well if you look at the volume 4 intros you could get the impression may be some form of copying, as you see one moment yang is fighting with Tai and the next moment in his place was a fully "armed" Yang. Well at least that was the impression i got anyways.
That has nothing to do with Tai's Semblance, though. That was all Yang.
It was meant to represent Yang remembering her old self before Adam cut her arm off. If you watch closely before that, Yang starts to hold her head and Tai looks worried and reaches out to her. The Yang with two arms is somewhat transparent like a ghost - a ghost of Yang's past. It's purely symbolic, not Tai's Semblance.