Central equilibrium is the basis of all the physical aspects of t'ai chi. Why is this so? If the body leans in any direction then the muscles must compensate for the tendancy of the body to topple under the force of gravity. You can check this out for yourself. Stand upright in a natural relaxed position. Cause the body to lean forward. Observe the sensation of the muscles tightening in the body to prevent the body from falling forwards. Release those muscles and then lean backwards. A different set of muscles come into play. When you lean you will also notice that the sensation of weight in the feet moves forward and back or even to the edges of the feet if you lean sideways. To achieve the optimum potential for the body to relax, the body needs to be aligned vertically over the centre of the feet. The three tan tiens also need to be aligned.
When the body is aligned correctly you will be able to sink the ch'i through the body. When you can sink the ch'i you will be able to separate full and empty. When you can separate full and empty you will be able to move without breaking the ch'i.