We're facing a redundancy problem. Our job involves quickly picking one combination that looks natural, rather than spastic, and which uses as little energy as possible. To demonstrate, look closely at your left index fingertip. Now think about the many different ways your shoulder, elbow, wrist, and knuckle could rotate and move directionally to make that fingertip go just where you want it to go. In turn, they also alter the movement possibilities for joints further along the limb.
Joints also move with particular speeds and in response to applied forces.
A fingertip can be moved to arrive at a certain place by many different combinations of shoulder, elbow, and wrist motions. It also can arrive at the same point in space tugging a heaving weight or barely touching a dewdrop. For He, each joint is mathematically described by sets of variables. These variables account for position in three dimensions and movement at different speeds and strengths. The tools he uses are mathematical expressions called algorithms.