The waist plays two roles when practicing Tai Chi. The first is that it transmits the energy generated by the legs up through the spine to the shoulders. It is also responsible for keeping the body in alignment as one body moves from posture to posture. This is why The Classics refer to the waist as the director.
One could probably fill a book with all the information the waist needs to know to be an effective director. But that would be silly because the waist can't read. Instead, I'll jotted some tips to keep in mind during your practice:
• The hips should remain at the same height throughout the form. If one hip becomes higher than another, the spine must compensate in an unnatural way to maintain balance. Try to visualize the waist as a bowl filled to the rim with water; don't spill a drop!
• The turning of the waist needs to be coordinated with the shifting of the weight. Generally speaking, in the Bow & Arrow Stance, the waist should begin to turn as the front knee begins to track over the ankle toward the toes.
• Be sure that the feet and legs give the waist enough room to operate. As mentioned in an earlier post, a foundation that is too short, long, wide or narrow will leave the waist little opportunity to keep the body balanced or move the energy.
• Be aware that turning waist too far can lead to tension in the hip and knee joints, or even uproot the feet. The direction of the navel should never point outside that of either foot.
• To achieve a solid connection between ground (earth) and the crown of the head (heaven), the abdomen should be slightly engaged throughout the form. By this I mean that the lower back is lengthened by activating the deeper muscle groups (most notably, the psoas muscles). This can take some time to find, but the reward is more than worth the effort. For example, when one's body begins to understand that it is the waist that directs the yin leg (whether that is to help close the back foot during a two hand push, or stepping into an Empty-Step Stance), one's practice will be so much more profound.
Consider the role of the waist in your Tai Chi practice as being similar to the hub of a wheel. If the hub is off-center or does not keep a tight rein on the spokes, the wheel will be unbalanced and much less effective. However, when the hub is strong and properly aligned, the wheel should be able to carry its load with grace and finesse.
Be the hub.