Interesting bit from " Running with the Mind of Meditation" by Sakyong Mipham
"The Psoas muscle behind the sternum and inner thigh initiates lifting the leg. Visualize the legs starting directly below the sternum in the upper part of the torso and back, for that is where these hip flexor muscles that initiate leg movement begin"
It gives a different flavour to your tai chi. The Waist is the commander!