Ron, thank you for links; bit too fatigued to look at them tonight but plan to read soon!
it's nothing that you don't know already. the chinese term, methink, is "kua". it's the power coupling area to transfer power from the legs to the trunk of your body, similar to the shoulder that transfer power from the trunk to the arms. ICMA would say to "keep the kua relax and round" so that you don't block or dampen the power flow to/from your legs to your trunk. personally, i think i have some blockage, because my ass is getting bigger (there might be a shelf or two here), yet my hips are still girlish, which presents a huge challenge to keep it out of the boot-up-the-ass-do.