The pinch to his nipple does more than just consciously encourage Castiel, as it makes his fingers tighten in Crowley's hair again, all on their own. He thinks he's getting the picture though; Castiel seems to enjoy the pain, so why not Crowley, particularly when pain is a special interest of his even outside of the bedroom. Plus, it's tempting to think about giving in to what his body wants, and his body wants him to let go, to drop the restraint and take something for his own.

Taking a firmer grip of Crowley's hair, he rocks his hips up intentionally, though not very hard. He's not quite sure what application of strength would be appropriate here, how much Crowley can take.