A variation on "silk reeling", folding the body at the spine... like opening and closing a door.
See the video here:
If you can stand with your 2 feet parallel and one foot in front with toe up, shoulder width apart and turn your upper body without moving the feet or the hips you can try to do this silk reeling variation. I explain later why this is a variation. This, like all moves in Classical Tai Chi is carried out by using the power of the abdomen and back. The importance of this move is to be found in the action along the strip of the spine. In this regard, the shoulder blade should not move. If just the shoulder blade is moving then the part along the spinal strip is not moving correctly. So, the movement will be concentrated along the spine. One side does move but irrevocably, you will feel some stretching on the other side of the spine...note the feedback from that in this instance.
Visualize opening up and closing a door which operates based on not only the action of the door but also needs the stability of the frame. Most people will find this move difficult to feel any sort of movement at that area of the back and along either side of the spine. For beginners, as with all movements of this sort, the difficulty lies in the mental discipline in which one’s attention has to be concentrated not only in the core region but to the specific area along the spine. Any attention on the shoulder blade or shoulder and the move will be partially initiated from those areas and not the spine. Done correctly and there will be no sensation of movement from the shoulder, shoulder blade or arm.
If we were to divide the body into quarters, the stable 2 legs and stable side of torso(that does not move) would comprise ¾ of the process. It provides support, power for the move and grounding. Since it is not moving it is Yin. The Yin-Yang junction is at the spine. The other ¼ is moving as a unit , arm, shoulder, back and portion of the abdomen, the hip or pelvis is stable on that side. That portion that is moving is Yang. One can also visualize that the spine is a very large hinge, with the “quadrant” or ¼ on either side (depending on whether one is moving r. or l.) like a door which can fold/unfold.
As to why this a variation: This is not the same as the classic “hand follows the foot, elbow follows the knee, shoulder follows the hip” that is described in the Tai Chi Classics. That deserves its own special place as a “folding” movement since it also involves the lower body.
With this variation as well, one might also visualize opening and closing a book since it actually has a “spine”. In this case, one would hold half the book steady including the cover, then visualize opening/closing the book repeatedly at the exact center of pages, including the other cover. Only half the book would be opened, the other half would remain steady and not moving. Of course we are talking of using just the top ½ of the torso and not the legs. As we have said, “hand follows foot, elbow follows knee…” has its own place in the lexicon of Classical Tai Chi.