Columbus Martial Artworks

An excellent article on exactly what the hips are. One part in particular jumped out at me:

The hips should always move as a natural consequence of actions initiated in the legs. Since they have no mechanism by which to move in and of themselves, it is an error to think that we move from our hips. Like a tree being swayed by the wind, the hips are caused to move.

As an instructor, this is a fundamental concept that I try to hammer home again and again. The legs drive the the motion of the hips, much like the transmission drive the wheels in your car.
Kudos to Mr. Marker for finding this gem.