This forces a specific bone hierarchy on the model, regardless of what's in the reference file. Normally, the bone hierarchy is generated based on the first occurrence of a parent child relationship. This commands allows you override it with any arbitrary hierarchy.
Note: The model builder will now allow the animator to change the bone hierarchy between animations, at least in the source. The hierarchy is unified into a single fixed hierarchy for playback, but it's often easier to change this for certain hand animation tasks. The hierarchy is defined by the first instance of the parent->child relationship.
$hierarchy (child bone) (parent bone)