Shape's expression -- a little dubious, a little possessive, more than a little hungry, and so, so laden with pent up lust.
Harry's expression -- wary but eager, afraid but brave, horny but uncertain -- he knows he should trust his captain, but he's just not sure he can trust himself in the face of all that discipline...
Uniforms. -- *Shivers in lust!*
Ankle buttons in shiny, shiny brass.
The allegorical lightning strike outside the storm-lashed windows.
The fact that there is no cane, no crop, no cat... just white linen gloves, a glass of wine, and a blush on the curve of that perfect arse. The firm hand, or evidence thereof, as promised by your title.
The glass of wine. Captain, and only the Captain can dish such a stirring discipline out without spilling a drop. Such poise... just makes you want desperately to rumple it. Which is, I'm sure, what got our Ensign Potter into trouble in the first place, eh? The price of his commission is nothing compared to the burden of his rank -- but from his expression, he's game to pay it, and do his duty.
This picture is well worth the words, and a million more besides! Well done!