A rig consists of a hierarchy of spaces. Your hand is generally controlled by the actions of your wrist, but if you put it down on a table, or on top of your head, or on a trapeze swing, it can become the primary pivot of the entire body, or a child of any number of different objects. One big challenge of rigging is how to manage the different parents and spaces of a rig and switch between them without issue. Hopefully this section will point to some solutions..
Broken Hierarchy and Space Switching (TD Matt)
Creating a Transform with Multiple Shared parents (p.i.m. thread)
another space switching pim thread - wrt writing a plugin
World space to Local space (Hamish McKenzie)
Breakdown of how to control an object in a hierarchy based on the actions of a different object, elsewhere in the hierarchy