The Master feels the grip loosening, and he takes that as a sign that he's already lost. Before any of them he can react, he grabs Andrew by the neck and slams him back against the doors, and puts the knife to his throat to make sure he doesn't move.

"Get off at the next stop," he tells Ambrose and the Doctor. Neither of them have moved, though they both jumped, as they wanted to run at the Master but resisted.