It sounds to me like it's mostly boosting and compressing the treble.
Not unlike recording a cassette (remember those?
) with Dolby NR turned on, and playing it back with Dolby off.
Yep, the old Dolby B pre-emphasis/de-emphasis. Which sometimes made commercially-duped tapes played on crappy tape players sound listenable. Not all the time. Sometimes.
Have a listen at it and let me know if you think it's doing what you think it's doing. It has knobs for threshold, frequency, mix, gain, and AIR, which is, I guess, "amount."
From this I suspect it's like it's some kind of expander, maybe? Like a de-esser in reverse? An "esser."
I'd love to have a starting point to figure out how it can be done with other processors. Just to hone my chops.