I'd guess that a sufficiently accurate sensor on your wrist could track the motion of your hand and fingers just via muscle/tendon/etc movement, for one thing.

I'd guess that a sufficiently accurate sensor on your wrist could track the motion of your hand and fingers just via muscle/tendon/etc movement, for one thing. (Place a finger on your wrist, and move your fingers around!) Combined with an accelerometer to track the overall motion, and you have a pretty versatile system.