Sup, stumbled upon your profile after me and a few friends stumbled upon all the halfcut{? detached? What do you call the head on legs thing?} posts on the Freedom Planet tag and now I'm genuinely wondering, how do you mess with the models to detach them from their torso?
I also kinda want to ask why alongside how but I understand if you don't wanna answer that one-
The way I get the models to look like that is with the help of some addons; Advanced Inflator Tool, Advanced Bone Tool, Invisible Material, and Sub-Material tool.
I'd spawn in two ragdolls of the same character, and equip the sub-material tool. The way the models have their textures called makes it easy* for me to make it look like they're separated. For this example I'm gonna use FP1 Lilac.
I would highlight one, and scroll to the textures and use the invisible material on them so that one ragdoll looks like a head, and the other ragdoll has their textures changed so it looks like it's just the legs, but I would keep the outfit's top material, which I would....
... First I would make sure neither ragdoll collides with each other, and then for the latter ragdoll that looks like the legs, take the advanced inflator tool and deflate the top half with a key combo, then use the advanced bone tool and get one part of her spine back to normal so it looks like the waist has been shut in.
Now with both of the ragdolls set to go, I just pose them for whatever scene I'm doing with the physics gun, and get the face posed and everything (and optionally use the 3D Textscreens mod if I wanna add in text), etc., and then with a small program called Flameshot, I take a picture of the scene with a delay so I can tab out and back into GMod, and then save it. The camera tool in GMod saves it in JPG format, but I use Flameshot to save it in PNG so there's none of that soft grainy look to it.
As for why I do this, on one hand somebody has to do it since these characters are rather underrated for this kind of niche, but on the other hand, something about this whole thing brings me some comfort (which ties into the first reason) (in fact this whole thing eventually led me to make some ocs of my own that naturally have this halfcut nature to them.) As for the term itself, it's just another term for SWB, or short waist body. I saw it floating around in some areas and slowly it just led to me using that for when I have someone in that reduced state.
If I didn't answer some part of this or some part wasn't clarified well, let me know and I'll do my best to answer it.
*On the GMod Workshop there's three different versions of FP ragdolls (as far as i know), one that was made for MikuMikuDance originally and then got ported over to GMod, another based on the FP2 look, and another that's called the Megapack, which brought in not only Lilac, Carol, and Milla, but also Neera, Cory, Spade, Maria, Magister, and some other characters.