The slight flinch tells the Master she's not necessarily indestructible. It's as close to any kind of upper hand he suspects he'll gain, regarding this strange new personification.

He also believes she has reason enough to inflict damage on him. After all, choosing one's words is his forte. The lack of desire for provocation does not amount to a lack of desire for revenge. The laser screwdriver remains in his pocket. For now.

"We're trapped in a finite space together. It's to our mutual advantage to refrain from tearing lumps out of each other. Now, if you'll excuse me. I'm going to have a very large scotch, and then I'll find myself a Doctor."