Well, for starters, there are a few ways to move joints. You can scroll, click, or press Z, X, and C.
There are 4 conditions each joint can be in: Relaxed, Held, Contracting, and Extending.
When you hold a joint, it simply stays still and does not bend(unless some kind of force is applied). To hold all joints, press C. To hold or relax a specific joint, press X.
Extending and Contracting is pretty self-explanatory. When you press Z on a joint, it will either extend or contract. So, for example, if you press Z on your knee until it says "Contracting left knee", it'll contract it next turn and your Tori will bend his knee. If you want to extend it, press Z until you see "Extending left knee", and it will straighten back out. That, actually, is how kicks are done in this game.