Hair Space Switch
Space switching are pretty common in rigging, and often they can be used to simulate gravity. If you have a hair control match the orientation of the master control even when the head controls moves and rotates, then it will look like it's being affected by gravity, and it's easy to animate some extra follow through because you don't need to counter-animate when you rotate the head. But most of the rigs that I have seen use this technique have a major issue. When you twist the head the hair controls do not twist with it.
I've done my best to make the solution easy to understand, but my implementation does use quaternions.
There is another variant of this technique that you can use for objects with a single degree of freedom, but I won't include it in this post.