To be honest, she expected him to make that grab, which was why she was holding onto the rope in the first place. If she was in his position, at the very least she would want to drag her murderer down with her. But she's not about to give him that chance.

He grabs onto her ankle, but instead of flailing her leg about, she braces herself on the rope she's holding and slams her ankle (and his fingers) against the corner of the plank she's standing on.