I had to google this as well. It looks like the idea is to move the surface that the key switches attach to, to a sub-assembly, which can then be mounted in a variety of slightly-more-flexible sub-assembly. This is the kind of minutae I would have loved to get into when I was younger. The closest analogy I can think of is a solid body (standard) electric guitar, vs. a semi-hollowbody electric guitar, although the mechanics are very different, ultimately you're attempting to modify the percussive effect by modifying the frame.

https://keyboard.university/200-courses/keyboard-mounting-st...

https://www.sweetwater.com/c592--Semi_hollowbody_Guitars