Many thanks indeed, Peter, that's a very helpful and well-written article, which I'm beginning to understand. I've printed it off, and will study it! And, clearly, for 'iceboat' read 'cat on foils'.
I was, of course, aware of apparent wind in a rather unstructured way, and had noted the AC cats tack/gybing downwind (as the faster asymmetrics do when racing). I think I've been hung up on - 'there's only so much energy to drive the boat in a given wind, so where does all that extra energy come from which is needed to make the boat sail so fast?' Of course it isn't extra energy, it's the energy already there but usually needed to overcome friction.
Amazing racing. And yes, as you say, hard luck on the Kiwis. However the 'national' status of both crews is misleading. I understand that Team Oracle USA (sic) comprised something like 7 Kiwis, 6 Aussies, 3 Dutchmen and a Brit!!
Michael