Does “Rotation in Joints (Separate but at the Same Time)” apply to other types of joints, like, as a less intuitively understandable case, the spine?
The spine is not one single piece but “consists of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx” (refer to http://en.wikipedia.org/wiki/Human_vertebral_column). The gap between any two adjacent vertebrae is bridged by an intervertebral disc. “Each disc forms a cartilaginous joint to allow slight movement of the vertebrae, and acts as a ligament to hold the vertebrae together.” (Refer to http://en.wikipedia.org/wiki/Intervertebral_disc.) In a broad understanding (excuse me for my poor knowledge in anatomy), the cartilaginous joints (probably except those within the sacrum and the coccyx, the latter being commonly known as the tailbone) could be viewed as joints in the ordinary sense.
Joints can expand; Idea can go to joints to cause them to rotate separately but at the same time.
In Wing Chun training, a learner is often instructed to keep upright, erect the spine. Instead of treating the spine as one long single piece and lifting it up by stretching it vertically straight, it is the “cartilaginous joints” expand on their own separately, composing and resulting in an overall spine rising upward.
It will become more visual when looking into the Darting Fingers Form-set (Biu Jee). In the last form, the upper body bends down along with the arms and then rises upright again, sending the arms upward and to draw sideway arcs downward for closing. In “bending down”, each “cartilaginous joint”, in its expansion mode, rotates forward separately but at the same time. In “rising upright”, each “cartilaginous joint”, still in expansion mode and already rotated forward, now rotates backward (appearing to be less forward) separately but at the same time.
Another example. There are considerable elbowing forms in Darting Fingers. In an elbowing movement to the left, for instance, each “cartilaginous joint” rotates separately but at the same time leftward. Although the rotations could only be just slight, the resulting movement of the spine appears to be a prominent “twist” giving out enormous spinning momentum.
If you can accept the above explanation, it won’t be difficult to imagine the same expansion-rotation operation occurred in all the other joints in the body. Say, the “articulating joints” connecting the rib bones to the thoracic vertebrae (enacted by radiate ligaments; refer to http://www.innerbody.com/image_skelfov/ligm30_new.html) readily expand and rotate (however slight) to involve the entire rib cage (the spine, ribs and sternum) in transmitting/contributing power to the elbowing movement. Yet another, say, the small joints up in the fingers and down in the toes.
Proposition #3: The skeletal system, activated with deliberate joint expansion and separate but simultaneous rotation in joints, takes over the master role from the relaxed muscular system on initiating every single movement of the body.