Mark - this is gonna be crude....but:One of the vacuum pots on the dizzy responds to venturi vacuum - the more air flowing through the carb venturi, the more vacuum applied. It should rise with steady rpm inceases - and increase the advance. After a fashion, it is providing the same feature as our later distributors with mechanical advance.
The second pot responds to manifold vacuum - provides additional advance when the engine is not accelerating (high manifold vacuum) - and cuts it back when the engine is accelerating (lower manifild vacuum).
Like I said ...very crude description.....