Love your quote -> “Sensitivity to ‘feet, hundredth parts, thousandth parts’ in practicing the forms is vital.”
Master Ni has said that the key to continuity is in the feet.
Paraphrasing: “Easier to have continuity in the hands, more difficult in the feet, but without the feet there is no real continuity.”
In terms of my own feet: I initially focused upon weight shift. As my awareness increased I began concentrating upon simultaneity between the bubbling well and the strike. Now with my focus upon 'step right - look left' I have begun to connect the alternate feet and hands with consciousness from the first point of contact of the foot. This also means a rolling onto the foot.
In this context Master Ni said that when stepping forward the heel leads - when stepping backward the toe. However when stepping sideways as in 'Wave Hands' the entire outer edge should be placed down simultaneously.
These subtleties have engaged my consciousness recently, allowing me to occasionally transcend the phenomenal world of existence to dance with the gods.
Thanks for your input. don