When I say that the compact upper body unit rotates around the central core of the spine, it means that the head remains in a fixed position and the hips remain fixed over the centre of the stance whilst the upper torso rotates back and forth in between the two parts
If you image a stake being driven through the centre of your head, through the neck, and down the centre of the spine until it hit the hips, the only way you would be able to rotate the torso, would be to rotate it around this stake.
