Sinister did not remove his hand from Billy. No, instead his right hand whipped up to grab Teddy by the throat as the boy came in fast. And without missing a beat, stopped the forward momentum and reversed it, tossing the alien hybrid back against the wall next to the door. Outside of his arm, the Doctor didn't move an inch.
"Such impertinence," he mused, squeezing hard on Billy's chin, making sure it hurt enough to make the boy cry out. His voice was clam, pleasant, "Should you attempt that again, I'll break his jaw."