Holmes inhales sharply, stilling his hand as he adjusts to having Watson's hand against his erection. Thankfully, Watson's progress downward had been slow, and Holmes had had time to mentally prepare himself; had Watson skipped the preamble, it's likely Holmes would have jumped and drawn away, his courage lost. He doesn't feel shaken now, or too invaded; it is very strange to be touched so completely, but it isn't unnerving.

What is setting him on edge is that Watson is quickly filling with lust, as he is wont to do, and that's part of why Holmes loves Watson so much, but that intensity of feeling is what set Holmes on edge earlier. He needs to remind himself that there is no danger, that he is still in control.

"Allow me," he murmurs. He finishes undoing Watson's flies quickly so that he can then undo his own. He takes hold of Watson's hand and presses it against his erection, making it clear to himself that this is something he wants.