The inside of the waist area is the dantian – it’s like the axle. Turning horizontally from a relaxed waist should make you feel like your legs and feet are like screws drilling into the floor, but not torquing the knees.
Now that you know what is meant by “the waist” and how to move from it in tai chi moves, you will gain much more power, structural integrity, peng solidity, and of course, qi flow.