Below is a nice video of Eddie Wu outlining the 9 joints of the body and their various usages within taiji. I was exposed to the “9 joint harmony” under the teachings of Wai Lun Choi, and the identifications are the same.
Arm: Shoulder, elbow, wrist
Back: Neck, Mid-back, Sacrum
Leg: Hip, Knee, Ankle
What I really like about Eddie Wu’s explanation is noting the difference in movement between the waist and the hips (kua). What’s the deal with the 9-Joint harmony? Well, as a teaser, try to think about this when doing the form and it’s relation to the 6 harmonies (wrist – ankle, elbow – knee, hip – shoulder)…