Heaven being the upper part of the body refers to the head and chest while Earth reflects to the lower part feet and hips.
Having a good Ma (Horse stance) and Ma Bo (footwork) is what drives and supports all our fighting tools in combat and without a continuous sure footing together with speed and flexibility we will not be maximizing our fighting capability.
Traditionally a good Ma was about standing our ground but to prevent becoming a human punch bag for our opponent one has to adopt a more flexible approach to our Ma Bo.
In order to maintain maximum mobility and stability, weight is even distributed down through the soles of the feet rather than through the sole and heel. This has the advantage of allowing us to spring forward or backward with minimal adjustments to our feet so increasing our reaction time.
So it is not necessary to be flat footed in order to maintain maximum footing.
For example when we are upgrading an opponent from a static position, it is about how we apply our Heaven and Earth core along center line and favorable arm positioning. This type of upgrading is incidentally should not be confused with the one where we press and lift off our hips. Although similar, the former does not require any downward or upward motion so concealing a much more insult attack on the other person's Ma.
When moving forward we need to visualize the chest and the hips converging to result in an apex of power. In effect we have two force vectors converging to an apex and it is at this point that we concentrate our forward momentum. This power apex also coincides with our own Dan Tian, a natural concentration of energy located at our physical center of gravity immediately beneath our navel.
With practice and training once we have isolated this area of power we should develop extra stability and power to our Ma and Ma Bo.