The slap, timed against expectations, breaks his controlled breathing into a quick hissed intake of breath and disrupts his amusement. You're supposed to be predictable, Doctor, even in this. Play your part.

"Yes, master." He turns his head, trying to catch the Doctor, so close to him, in a kiss. Moving his hands and arms in as he does so, he's poised to push up, to make the kiss the opening of a bid to reassert control, to tip the Doctor off and over.