User blog comment:Xilinoc/Ch. 567 (Rukia be like "I ain't even scared though")/@comment-5693502-20140205150657/@comment-5398287-20140206055010

The molecules of her body were no longer moving at that point, or somesuch. If an object reaches a temperature where its molecules are no longer moving, it's completely impossible to move or penetrate said object with an outside force. And before you ask, Rukia was able to move her body because she was manipulating the Reishi it's composed of.