Tai chi is full of opposites.
We talk about hard and soft, ful and empty, up and down and so forth.
Here is an example of left and right in the waist., using Brush Kneee and Push posture. As our weight shifts from the right back foot to the left front foot, the waist turns to the left. As we arrive on the front foot our waist should also be pulling back to the right. This produces a torque in the waist and has the effect of strongly rooting in the legs and feet.
The waist then unwinds as you transition into the next posture. The waist is constantly winding up and unwinding. Full and Empty, Hard and Soft.