"JPEG, without recompressing" is just does the same lossless JPEG transforms as most lossless JPEG editing tools, transforming and reordering MCUs, and dropping those that are cropped out.
There's an option of whether to expand or contract the cropping area so it matches MCU borders. I did want it to be possible to see directly in the GUI how that would work, but I never figured out what the proper way to implement it would be, so I never got it done.