The Mistress sees in a flash his plan.
The Master has not yet become so familiar with this version of the Doctor to understand all the tiny snippets of information in his head. He has not yet learned to think like the Doctor.
So she hits him with just enough force to crash against the keyboard.
She feels his success in her mind as he passes out.
The Master smiles at her as if he has won.
She doesn't disabuse him of the notion.
The Mistress has already decided that her past self is flawed. Why would she have started changing if she hadn't?
Why would she help him now she has?
