One of the concepts I was exposed to in Functional Range Conditioning was called CAR's. Controlled Articular Rotations. Without going into tremendous detail, joints should be able to do circular motions pain free and can be trained within the system.
Dr. Spina talks about a morning routine that is done where these CAR's are used to use and explore daily motions. If a joint can move pain free through a circular motion it can essentially be validated healthy.
Joints that can rotate in full ROM pain free are by definition pretty healthy. How often do we take a shoulder or neck into full circular ROM daily? How about a wrist or ankle?
Going back to Leonardo's Vetruvian man where a circle represents perfection, there is some underlying human anatomy that is truly only explored through rotations.
Listening to Charlie Weingroff talk about circles and rotations was very interesting in the Lateralizations/Regressions DVD.
It's interesting that rotating the ankles clockwise/counterclockwise several times will improve hamstring flexibility. Rotating the thumb improves wrist dorsiflexion. These are just a few examples.
Rotations allow you explore movement that you might not normally go into. This may be the key to it all. Driving synovial fluid into spaces that don't normally get used.
Scott Sonnon has a very interesting strength and conditioning technique called Circular Strength. Some of these I've adapted into my own practice with Clubs.
For a few days practice moving your joints in circles/rotations, explore movement and pay attention to where you feel less tension.