For me at least the essential hotness in this one is absolutely clear: the lowering of panties has an almost mystical quality for me, many times. It's very strange to think about how recent a development this close fitting undergarment actually is (less than eighty years old in anything like its present form) and yet how very, very fundamental to so many of my (and, I'm pretty sure others') fantasies.
Panties, in my fantasies, never go around the ankles unless a Master has so decreed. A Master so decrees because he know how it makes his sub feel to have a constant reminder, around her ankles, that she has had to pull her panties down (and of course she has had to pull her panties down because her Master told her to pull them down).
Why did he tell her to pull them down? To enjoy the view; to get access to her backside for discipline, should he wish to discipline her; perhaps more than anything else to make sure she knows that her charms, front and back, belong to him, and that they will be exposed in his presence to signify that she has no right to cover them.
Incidentally, I think that's almost certainly why lace panties in general are my go-to garment for my paricular genre of BDSM fantasy-elaboration: they are a veil that never ceases to show. It's a very small step, given how overdetermined by culture our garments are, to imagining that that false veil has been imposed by an authority-structure (or, in more common and specific terms, a Master): and it's of course that placement of the object (the girl--me) within the authority-structure, her modesty always about-to-be violated, that makes everything hot. As always, it's not the garment; it's what the garment means. (So too with the thong--the covering that isn't a covering.)
The thing that turns that essential hotness up a notch for me is the wood floor. Where are they? I thought of several candidates, but I decided to go with the unfinished look of it so that I could add in the carpenters watching. I suppose one could argue that the carpenters aren't implied by the picture (just as the spanking might not have been implied by the lace and chocolate last week), but after some thought it really does seem to me that a key part of the picture's special charm is the way that floor (and the light contributes greatly here as well) seems to speak of a place more rustic and more exposed than where you would expect a girl to be ordered to take her panties down and get to her knees (say, a bedroom or even a living-room). I hope my carpenters bring out that quality.