This play of double entendre they're acting out is convenient, and enjoyable, and helpfully allows Holmes to reveal while concealing. It's his line, now, and he thinks he can spot his cue, so he snakes his hand down between them, cupping his hand over Watson's erection.

"So do I," he says darkly, and it could be that he literally feels Watson 'improving,' or it could be that he feels himself improving -- either, both.

The heat against his hand is curious. On the one hand, it's slightly alarming to feel Watson's physical desire for him, right there in his hand. The rest of him, however, responds with a carnal enthusiasm to feel that familiar weight, to be returning to these activities.

His heart pounding ever so slightly, either from nerves or eager anticipation, he begins to undo Watson's flies.