He holds still a moment, groaning in an uncharacteristically loud way for him, though really it's not so very loud as... well, Watson. Collecting himself, Holmes takes a gentle hold of the base of Watson's cock with one hand and braces himself with the other.

This position puts Watson's cock in an unfamiliar position in his mouth, and he matches Watson's unhurried pace so he can learn this new feeling and adjust appropriately. Of course, this means he experiments; he takes Watson's cock as far as it will go, then slowly releases it. He sets a pace, and interrupts it to taste the head, to kiss along the top of his shaft, only to find his rhythm again.