Three parts in ball bearing: outer ring, inner ring and the balls in between we consider as one part.
Taiji is three.
Three steps: 1. Different parts of the body are strong, hard and stiff, which able to combine and recombine into pieces which have the same quality, like the rings and the balls. 2. Establish relationship or connection between different pieces through joints, like the balls seamlessly in between the two rings (through separator and tracks on the rings), engine oil is the joints. 3. Press the button to run the machine.
In the ball bearing, the three parts are doing their own things independently at the same time within their own firm structures. Movements do not contribute to the structure.
Similar example Master Chen Zhonghua has given. Car system: there are the engine, fuel system, transmission, electrical system, water system and so on. Each system cannot leak into or affect any others. But together, they make the car work.
For Taiji training, we need to be able to combine and lock different parts of the body into firm pieces, and only rotate the joints. Once the connection is established, the hand, elbow, shoulder, kua, knee and foot will be moving on its own trajectory and dimension proportionally at the same time, while there is only one button to press, only one active part.
-Kua exercise. Lock the arm to the torso as one piece not moving. Push the front kua out to drive the hand out. The torso will lean back to balance. Make sure the part from the head to kua is one piece, the neck and shoulders tend to move or adjust on its own.
-Elbow exercise. Place the whole arm on top of a table. Shoulder is against the edge of the table. Lock the hand, wrist and forearm as one stick. Bend the elbow. Use a stick or a physical line to mark the line of shoulder and hand. Push the elbow 90 degree towards the line and the hand is propelled out along the line.