Vertical Axis
The body must be trained to have multiple axis and they must be in multiple dimensions. One of them is the VERTICAL AXIS. Vertical axis acts exactly the same way as a door hinge.
On a door, the door hinge is vital but the actions of the door hinge is difficult to be observed. Same is true in taiji movements.
[hdplay file=/videos2011/VerticalAxis.m4v hdpath=/videos2011/VerticalAxis.mov ]
Source: https://practicalmethod.com/?p=17391