User blog comment:Xilinoc/Ch. 567 (Rukia be like "I ain't even scared though")/@comment-5693502-20140205150657/@comment-26420452-20140205163906

I interpret it that she freezes her outer body, similarly to what Yamamoto does with Nishi: Zanjitsu Gokui, and has to manipulate her Reiatsu so that the freezing doesn't penetrate to the core of her body.