Greetings..
Aligning center of body-mass over yongquan will set the spine off its perpendicular relationship to the pull of gravity.. the spine will be oriented several degrees to the rear, such that any activation of energy from the ground upward will initiate a movement oriented to the rear/retreating.. Setting the spine in the vertical alignment, perpendicular to the pull of gravity, is the neutral position.. even-though the body-mass is slightly forward, it is also oriented in the direction of our attention.. this alignment shifts the weight from the heels (static) to the 'bubbling well' (active), but it does not disconnect the heels.. the heels become bracing supports.. When bending the knees, shift their position forward, over the toes.. this will naturally rotate the hips and orient the sacrum downward without having to engage the abs.. The legs will support the relaxed upper body in a most unique way, with the spine vertically over the sacrum, gravity pulls the pelvis into alignment while the upper body relaxes into structure.. the gentle pull of the 'body-mass forward' actually opens up space between the vertebra, and sets a natural 'bow'..
Now, as i have learned, the real trick is to incorporate this alignment into our Taiji..
Be well..
TaiChiBob.. "the teacher that is not also a student is neither"