Ben
That exercise was taught to me by a master of Tai-Chi and shaolin
.and it is a TC exercise. There are other TC exercises that may help.
stand in a horse stance and circle your waist ( like using a hula-hoop, do both right and left clockwise and anti-clockwise, bend your knee and "push" off your back leg.
bearing in mind that I am only a beginner asking questions and not a master offering solutions
...........a couple of things spring to mind, firstly you are grounding yourself by doing this pose i.e. Sanchin, you are making yourself a conduit to the ground............rather like a spear stuck into the ground, as in olden times to stop cavalry.that is why you must "
Feel" your heels, and also why you must not move.because if you do move or don't feel your heels you are not a conduit and you are doing it wrongly
.........also this could possibly explain the Zenkuts stance resembles the TC stance but is done differently......in TC it's done like you are pushing a car....so your power is being transmitted through your legs from the ground and your arms play no part, so maybe...this type of grounding can be practiced in any stance
...............If you get in this stance and extend one arm and do the exercise again, perhaps
also I now remember an Aikido exercise were you row a boat.....you know one of those Japanese ones where you have to stand up and sought of push forward then pull back?.......well in that exercise, at each extended position your are supposed to be able to withstand a push
...........so I suppose if you can do that in Sanchin it would be good, also check out
http://www.aikidojournal.com/encycloped ... ntryID=197
and look for stuff on Goza Shioda..........he's a tiny,little guy ( or rather was
) and he used to bounce big guys off using that technique.some good clips of him there.
Also one thing that Rick said which lit a lightbulb in my head..and this was that Zhing-I was spearfighting..so you have to think of their punches as spear techniques and therefore you have to have that grounding element as well
...hope that helps
to be practicle I would suggest doing Sanchin against a wall left and right for minutes each side then with one hand extended into the wall each side then maybe just standing "like a tree" arms rounded fingertips touching,legs bent,back straight.for 5 minutes.after a month get a buddy to push your shoulders when you are in Sanchin ( hard)..........see how that grabs you, treat it as a meditation
...there is a lot of this type of thing in "Good" Tai -Chi..but it's very hard to find. My own teacher is quite a strong fit man ( although middle aged.....a Grandparent in fact 
).............from a TC perspective, get into your Sanchin stance while facing a wall, place both palms on the wall as though pushing it, and in fact do just that....but try to feel that the push comes from your heels.you should feel pressure on your heels and palms..but you shouldn't push yourself of the wall.in fact you should remain rock steady, and in TC terms you will feel "grounded".....you can use that feeling in stand up grappling, chi-na type of stuff you will press forward more with your energy....you do this a whole lot more in WC.........in fact you become a "space invader" 
........because you actively invade other people's space.but Jim knows a lot more about that than I do 