"Don't get excited."
He should be scared, but John Watson has long since realized that while he is a compatriot of Sherlock Holmes, situations like these (even ones involving very sharp blades and pig parts) will be commonplace. What is atypical is the addition of the lady on top of him. Miss Adler, who is after all, the only one in true danger.
Usually, it would be Watson himself strung up, waiting impatiently for Holmes to think of some brilliant maneuver to save him from his untimely fate. As it stands, he is of course worried for the lady he struggles to support, who unfortunately will not stop wiggling, but he is also somewhat... put out by her presence. The thrill of the chase, the gunshots and adrenaline, the sight of Holmes's heroics, all of these have left him somewhat… animated.
It does not help when, as part of his strange rescue efforts, Holmes reaches down and undoes his belt. Watson is ashamed that, preoccupied as he is, he still shows evidence of his arousal. Holmes smirks as he pulls the buckle apart and smoothly pulls it free, mutters a, “Don’t get excited,” all the while implying with his expression the unspoken, “until I can truly take care of it.”
The moment is interrupted as Adler squirms on top of him, as if to say, “Stop flirting and save me from my untimely demise.” Holmes rushes to oblige her.
Watson unfortunately remains frustrated, of course, until long afterward, in the aftermath of Holmes’s confrontation with Blackwood and his own struggle against the French giant. He is pleased to note, however, that at that time both he and Holmes are quite… excited.