@bod8 @blaze I've trained joints to crack and to uncrack. I actually kind of regret the uncracked ones, but since so many people in the forums here dislike their cracking I'll share what happened to me... The more you crack a joint the easier it becomes to do, and vice versa.
One is my left knee. At one time my knees were the easiest joint to crack. They usually had "a crack there" (my term for when you can tell a joint will pop if you move it right--I think my body can tell there is air there), and even if they didn't I could "set them up" by bending and unbending my knee while pulling backwards on it very lightly with my hand such that it was now crackable. I like cracking my joints but I felt like I'd taken the knees too far... they cracked every time I straightened them, no matter how little time had passed. (People say sounds that happen that often are ligaments popping, not air pressure released, but it was definitely a true crack; the cracks have been the same my whole life, just more or less often, and they couldn't pop continuously, it was a pressure release, it was just that they could fill again after about 10 seconds. I also didn't need to manipulate them very far or fast to pop them anymore.)
I ceased intentionally cracking them. I also avoided movements that made them crack: I would unbend them, stand up, etc very slowly so they wouldn't pop. That was the hardest part because most movements made them crack, but it's doable. My knees fill up when they're bent, so I would keep my legs straight whenever possible. You know that feeling when you try to crack a joint, and it almost goes but not quite, and is now impossible to crack? I usually find it maddening but I did it on purpose to my knees sometimes. They improved after not very long. My right knee is normal now, I crack it maybe 0-3 times a day, and my left knee is hard to crack and almost never fills up on its own, I have to do the hand trick.
The 2nd joint on my thumbs were similar where I could crack them every few seconds. Same thing, definitely a genuine crack, I can just crack them up to 3 or even 5 times before all the air is gone and they fill up fairly quickly between rounds. I have a harder time explaining what I did with this one... I was trying to crack the right side and I knew while I was doing it "don't move it that way, you'll screw up the crack" but I wasn't careful because part of me thought that wouldn't be such a bad thing... it almost-cracked and was gone, but the weird thing is, the joint's been totally different since! It used to match the left side, which I can still get every few minutes. I'm hoping cracking it normally will fix it but no luck so far... I can only get it when I wake up and stuff now and I have to be careful. Actually I think that's what part of sealed the deal though-- you untrain the joint by doing the movement that usually makes it crack without getting the sound, which is what I accomplished by trying to pop it so often.
The last is my back and neck. I stopped cracking them because my neck was kind of hard and I need to brace myself against a chair for the back. I used to be able to get them if I tried; then I went a few years where I almost never tried. I can often still get my back, but only once or twice in the first direction I twist, and it feels very stiff.
I definitely think there's more to training joints than just the ligaments.