To other men (and women, he supposes), in other situations, this act might be degrading or entirely displeasing, but under Watson's attentions, Holmes feels... treasured. The benefit to this position is that less concentration is required; he drags his tongue in patterns against the underside of Watson's cock, but he also has the opportunity to take Watson in as he indulges himself, a task that's much more difficult when he's in charge.

His hands rest still on Watson's hips, and he strokes his fingers over his hipbones slowly as Watson experiments with his pace, but he's too impatient for too much gentlenesss. He slides his fingers over and presses his thumb into the mark he sucked onto Watson's hip.