From this post:
Joints need to be oriented with one axis pointed directly down the joint, towards its child, and another axis pointed along its main rotation axis.
The documentation says I must have one axis pointed toward the tip of my finger and one perpendicular to the joint. The picture shows the third axis going down/out the palm, but doesn't explicitly call this out as required. Is this third axis required to face that direction? What will happen if I make it out the top of the joint/out the back of the hand?