The spine is a major energy path that deserves close attention. After the common cold, back pain is the most frequent ailment reported and it is often caused by tension due to poor posture. The center leads the movement of the energy and the body. The waist area just below the navel is the central axis or hub of the body and the limbs may be thought of as spokes. Paying attention to the center on the physical level gradually leads to the meditative aspect of T'ai Chi as well.
Breathing is deepened and enhanced by concentrating on the center.
Differentiate yin and yang and perceive substantial and insubstantial. Initially, this means to separate the weight completely in the legs. T'ai Chi is called one-legged boxing because of this emphasis on standing on one leg at a time. Obviously, this builds strength and balance. Gradually, one becomes more sensitive to other differentiations as well.