She's using her body to guide his, and he has to brace himself with both hands because he needs to move faster, harder, and she's the one watching him now--

His mouth drops open; his eyes close. His hips jerk and tremble uncontrollably, and he makes a choked little sound of surprise.

Sherlock comes harder than he ever has in his life.

It's less like the small, functional orgasms he's used to and more like a grand mal seizure (or at least what he imagines those must be like). He can't think. He can't breathe. He just feels, and it floods his brain with light.