More or less you have the right idea. Diaphragms or air baffle plates actually prevent airflow through the secondaries until the velocity is high enough to pull fuel through the secondaries. This prevents what is commonly called "bogging" at WOT. Since both systems will open gradually as airflow increases this brings the secondaries in as the engine can use them.
A diaphragm like Holley uses can be tweaked by changing the springs under it to cause it to open sooner or later relative to manifold vacuum. The air baffle plates need their counter weights or the weight of the plates altered to do this. Generally speaking this is not needed on a street driven vehicle and the only correct way to alter the plates is on a flow bench. Drilling or reshaping the air baffles is a one way street and once you alter them you cannot go back, the plate will need to be replaced. The AVS Edelbrock has an adjustment to alter the plate opening and is easily returned back to the factory setting without having to obtain a new plate.
