It was obvious that Black was either going to ignore him or blow up at him over nothing at this point, which didn't give Severus much incentive to contribute further despite the fact that he was the one who knew how the spell worked and felt from the caster's end.
Besides, Remus had promised him lots of shagging any way he wanted it if he held his temper during the lessons, and he didn't want to jeopardize his chances when he thought he'd done well so far. Thus Severus sipped his tea, thought about new and inventive things to do with the shibari ropes, and said nothing.