The progression of tai chi focuses attention deeper with continued practice over time (gongfu) on the inner alignment of the body as represented by this diagram. Beginners focus on the outside movement of extremities, while more-seasoned learners are able to focus more deeply on the core alignment and how to move there.
The three dantians: lower (abdomen), middle (solar plexus), and upper (forehead) are key loci for moving from the inside. Beginners should hone their focus on them during practice. The result is the external moves with the internal.
In tai chi terminology, wai dan is external and nei dan is internal. The “whole body moves as a single unit,” which is considered more effective for achieving many desired results. If you view wai dan and nei dan as fields of energy (dan) you will be going even further beyond physical movement into another level of skill and ability.
I a future post I will talk in more detail about the progression of awareness in how you move the body from the extremities to the inner structure.