She might be trying to use Yuuka's power to destroy her ribbon. If that is the seal to her true power, and if successful of removing it, Yuuka may be in big trouble.
More like Yuuka will gain a new drinking buddy if you ask me. Kind of like the Takamichi Nanoha approach to making friends, only a thousand times more bloody and brutal.
With Rumia upside down, and Yuuka facing the other way, but clearly looking ready to kill, Rumia may have just leapt over an attack of yuuka's, up and over her.
