One of the things that took me a while to understand was changing the force interaction at the point of contact by starting my movements from joints relatively distant from the point of contact.  In the beginning, my first response was to change at the arms (i.e. moving primarily from the elbows and shoulders) since that was where the point of contact was.  My arms also happened to be where I had the most motor control at the time.  Of course, the problem with moving primarily from the joints close to the point of contact is that those joints may not have the necessary degrees of freedom to move while maintaining the sphere of defense and maintaining the unified body.

After repeated instruction and practice, I managed to get the feeling of the body moving back to expand and drive energy more forward.  I also began to feel how the torso shifts forward slightly to condense and draw energy in.  It was a little counter-intuitive at first.  For the longest time I only understood this as a way for maintaining the stacking over the center of the feet.  If the arms draw in, the center of gravity shifts slightly towards the heels, so the body moves forward slightly to maintain the weight dropping to the center of the feet.  Likewise, if the hands expand outward, the balance shifts slightly towards the toes, so the torso moves backwards slightly to maintain the center of the feet.

It only recently dawned on me that there are other mechanical reasons for shifting the torso position.  Few things in the body operate independently since the body is so interconnected.  Changing the torso position moves the shoulder position, which in turn moves the elbow position relative to the wrist.  That alignment change between shoulder, elbow, and wrist is in itself enough to initiate change in the nature of the point of contact.  Assuming a wrist point of contact which stays relatively stationary in space, moving the body forward moves the elbows outward such that the elbow and wrist are better aligned for left-right force.  Conversely, shifting the torso back draws the elbows in such the elbow to wrist alignment better expresses forward-back force.  Dropping the body down lowers the elbows so that the elbow to wrist alignment better expresses the upward south energy, and raising the body up moves the elbows higher relative to the wrists such that transition to the downward north energy is facilitated.  While it is a bit of a simplification of the process, the model of the joint changes was a pleasant realization.