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Mike,
My understanding of this is different. To illustrate (taking great technical liberties for simplicity):
If fuel always burned at the same rate, and you always want the peak of the burn to happen at TDC, you have to light it off increasingly early as the engine revs higher. This is handled by the centrifugal advance in the dizzy. Simple enough.
But fuel does not burn at a consistent rate. The more charge you put into the cyls, the quicker it burns. It takes an increasingly big charge to make the engine spin 3500 rpm or higher under load, so there's no need to keep advancing. This is why centrifugal advance tops out at mid-rpm -- beyond that, the burn rate makes up for it on its own.
But what if there's not a lot of load? Doesn't take much charge to go a steady 3500 rpm in third gear, say. In that state, the burn is slow but the piston is moving just as fast as in the previous example, so with centrifugal advance alone, the max burn happens way late.
That, of course, is a high-vacuum condition, and the vac advance is used to bring the timing up to where it should be. Under low-vac conditions, like wide-open throttle, it does nothing, and it shouldn't do anything. Max performance is unaffected by vac advance, but part-throttle efficiency can be greatly enhanced -- you get a lot better fuel economy at cruise.
--Phil
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