In a recent post by D. Mroz (a classmate of studying with my teacher’s teacher Chen Zhonghua at Daqingshan in the spring of 2007), he quotes Richard Backus’ notes on training:

As we practice the circles, or use them in the form and push-hands, we use a
hinge-like action, with the hinge pin alternating from one hip to the other.
i.e. When drawing in, the back hip has the fixed pin, while the front hip is
like to movable leaf of the hinge. Then, once the circle begins to project
out, the front hip has the fixed pin, while the back hip is movable.

 I have been trying to do this for some time now.

I also saw an interesting video that (I believe) illustrates the point.

Around 1:30 he explains how he places two stones under the massive concrete blocks to move them. I think the kua functions as the concrete block and the two stones function as the connection between the legs and the kua. (I need to explain more here…)