Clicking a joint will change the state of that joint.
Different states are:
Relaxed
Hold
Contract
Extend
Rotate left/right [chest]
You can press "C" to instantly hold all.
You can press "V" to instantly grab hands.
The goal of the game: Deal damage to your opponent by striking them with your hands/legs, or whatever you have at your disposal.
Check out the Wiki for some moves.