Twist bone
A helper bone that distributes a limb's twisting rotation along its length, stopping the "candy-wrapper" collapse that skinning produces at wrists and forearms.
When a forearm rotates 180° around its own axis, naive linear-blend skinning pinches the mesh to near-zero volume halfway down — the candy-wrapper artefact — because vertices blended between a fixed upper bone and a fully twisted hand bone cancel out.
Twist bones solve it by inserting one or more intermediate bones along the limb that each take a fraction of the roll (a bone half-way down = 50% of the wrist's twist), spreading the rotation so the skin deforms gradually. They are usually driven automatically from the child joint's rotation rather than hand-animated. Standard rigging practice for arms, thighs and necks; dual-quaternion skinning attacks the same problem differently.