Paul,
The body works along longtitudinal chains of muscles (which incidentally, is the basis of Chinese acupuncture and meridian system, so when Hendrik uses the 8 extra channels or
Qi Heng Ba Mai as an explanation is not far off), and basically, through a lot of practice and kinesthetic feel, you will get that right linkage.
Basically, you are looking for that positioned "wedge" or "sweet spot". For example, when shoveling snow or dirt, you have to "get just underneath the handle" to apply the leverage, and then the shoveling becomes easy. The structure test is to align the body vertically to make you into a human "wedge", just equalizing enough first then be able to ward off, redirect, guide, totter the opponent.
Some muscles are relaxed, others are contracted, others held with partial contracture. Muscles relaxed will give you more weight. Too much muscle contracture will make you a lever to be easily tossed. Relaxation and letting go can help a lot here.
The skill is in the practice and the kinesthetic feel. And of course, that requires a lot of time to develop skill.
I don't know how much that helped because really, kinesthetic feel is required. I tried my best here in this medium, Paul. I think you owe me a pretty girl poster.