So I figured I can give my opinion on this, from a general view as I did try most of the competetive modes, and fiddled around in replaymaking.
I see no reason to implement another motion, as when it comes to replaymaking, you could add motion by extending a joint, and after 1-5 frames turning it to relax, which will use the move already set in motion, but will act out with less force, unlike when you'd continue the extended motion. Same goes for extending/contracting and then holding the joint, if the dynamics allow the joint to remain held, it will remain in its original state.
I can't really be bothered to showcase what I mean, but most replaymakers, especially people involved in parkour, will agree with me that there already is a "motion between extending and contracting", and it's lowering frames to input a hold/relax within the move.
So that's why I don't see any reason for it to happen in that sense.
When it comes to a comp meta standpoint, holding and relaxing is already enough to really work your way around many things (ie relax your foot if you want your kick to spread out further, hold/relax your wrist so it doesnt dm because of the angle). While the idea may make some sense, I really believe the toribash meta hasn't yet fully used the potential of what we have, there are still new openers and strats being made, and although we continue to play the same mod rotation (abd/boxshu/lenshu/tk), there is a lot of space for new mechanics.
To just finish up my argument against the new "joint motion", I'll bring up a recent update to the meta. The new grabs that were introduced by sir are definitely a nice addition, but they haven't shifted the playerbase to play the respective newgrab versions of our standard comp mods. I've seen some replaymakers use the new grabs in their parkour maps, or even made new mods related to climbing (very neat stuff btw)... but the core element of toribash, so the comp scene, really prefers the old grabs.
So if we look at it from the most realistic viewpoint, this new feature would only be used by replaymakers, and they already can imitate any progression motion between extend/contract by simply using hold/relax.
I like the suggestion, but I personally don't see any reason for it to be implemented.