Shioda talked about it a lot, and quite clearly, as in "In order to generate Kokyu you must stand as if you are making your big toe sink into the surface of the ground." - he often spoke about the importance of bringing power from the ground. There's a video clip somewhere where he says it straight out, hesitates, and then changes the subject.
I was lucky enough to get the "big toe" instruction right at the beginning of my aikido training and it became the default. This was different. The instruction for bagua was to make a connection right from the end of your leg, as if your leg bone was rooting into the ground. The whole foot is placed as a unit, and the front part of the food has a "sticking" or "grabbing" feel to it. Of course the toes and the heels are connected as well, but the primary intention goes to the middle of the foot and grabbing part at the metatarsus (could be wrong on the terminology - I'm not a kinesiologist or an art student).