His hands might be bound but he still manages to get fingers on Sherr's shirt, gripping them when he squeezes tighter on his neck. Jim's mouth falls open and a feeling of wooziness hits him, a sound caught in his throat that only partially makes it out.
Although it's a turn on for his body, his mind is enjoying the turning off it allows. Jim Moriarty, the man always plotting and always in control, sometimes just needs to turn his brain off to give it a rest. The corners of his open mouth tip up into a lazy smile at this, the ease of it.