What in your vision of a great pianist is the 'foundation' to their ability?
Generally, I would equate 'foundation' with 'discipline'. That knowing what to do will be useless without the discipline to do it. Discipline then becomes the will to not allow your ego to 'bypass' the work required to develop a technique. Reaching the 'music' as quick as possible, by the shortest distance possible, even at the cost of NOT having developed anything, IMO, is the ego interfering. We then wonder why in the spotlight our facility fails us. We break down, the music slips away, our nerves overcome us. Is it not always the same? It is in that moment that we realize (or not) that we have not done enough.
IMO, foundation is not really 'technique', but rather the means by which it was developed. The discipline to train ourselves how to practice. Teaching is like the task of an architect. Once u have drawn your plan, it must remain open to changes, however, aspects must be followed to a tee, and it is a matter then of seeing that that plan is adhered to. The foundation is from where all else grows, but what is it?
Across from my room, i have had, and continue to relish in the sounds of a construction site. I look out my window and have watched the excruciatingly slow progress of the foundation to a condo. It has been six months from 7am-midnight every day. And all there is now is a foundation. Nothing to look at, it is ugly, an eye sore, noisy, it bothers everyone, etc. But it is relentless, and necessary. It must be done. The ground must be excavated, moved, molded, melded with the earth. Long pilings of wire and cement were driven into the ground all over the area. It was really unbearable. I pitied the beautiful vacant lot which was being destroyed.
One possible problem with such a metaphor, or trying to know what a foundation to piano playing is, is that maybe there is no 'foundation' at all. I also like to think of technique as a kind of cicular self reflexsive living system, which feeds back and develops in an incremental way. Like real living systems. Hearing in your mind, playing, and listening all interweaving, developing towards a deep inner necessity which guides, moving towards an unknown destination.
I was told by a friend, that a student came to him and asked to be taught how to play Ondine. Well, he said, do think u could first just play me a b major scale? The std went to the piano, and rippled off 4 octaves...but, ahem, (with the LH an octave behind the right). This friend said to me, that his reply first of all to the std was that he has no 'foundation' to his technique, or in other words, that he has NO real technique to speak of at all. So Ondine, therefore, he recommended, may not be the best piece to start with...