In taiji, an important concept is to be rooted. However, I don't think this means being firmly stuck to one place. Rather, it is about being able to translate all actions such that they begin and end from the ground. At the same time, this "ground" is anywhere that the body contacts the natural surrounding.
The key is not to become a rigid object that is in contact with the ground. The concept of being rooted is to derive strength from the ground, but the translation of that strength to the application point requires the movement of the various joints in as a complex series of linked levers. Taiji theory helps explain this using the major joints like the knees, kua, waist, shoulders, elbows, and wrists. When practising taiji routines, it is thus important to have a mental image of how the movement of each joint is in series, how they work as a single move in total, and how this entire move is linked to the ground.
In a more simple way, it is about visualising in the mind the entire path taken by the force, originating from the ground (which is where the person, usually the feet, is in contact with the ground) and traveling through the rest of the body until it reaches its final place of application. And I think it is also important to realise that force can be applied anywhere along that path. This thus adds flexibility to the application of taiji moves.
