i thought Irene made it pretty clear that you needed to exert self-control in order for the Flash Sword technique to not awaken the rest of your body... and I felt that that was largely a mental thing...
I guess Clare was trying to say that she was surprised that Jeane could keep her mind human when her entire body had Awakened, and so now Clare can Awaken her arm entirely (without fear of that starting a chain reaction) and thus has a better flash sword tech.
in short, what bud said...
edit - also, and more importantly:
Nipponsei - Claymore - Original Soundtrack