Dennis, as well as others, let me take a shot at explaining the Holley quote,
I read it as when vacuum has fallen, to some value less than the number stamped on the valve, it opens to allow more fuel to join other fuel provided from another circuit. This "collected" volume of fuel doesn't do anything until acted upon by atmospheric pressure when presented by the opening of the throttle plates.
Engine creates vacuum below the throttle plates. Atmospheric pressure tries to fill the void created. As air is pushed through the venturi, a high pressure area is created, thus draws fuel from the well and the fuel is atomized into the stream of air by the high pressure created at the venturi.
Therefore, I believe the secondary power valve would not have an effect on idle characteristics until the secondary plates are cracked(?) open.
I am not a master fuel technican, nor a physicist, just my two cents......
Everett "OBJECTS IN THE MIRROR APPEAR QUICKLY UPON RAPID DECELERATION"