The Holley vacuum secondary carbs have secondary butterflies that are operated by the vacuum developed in the primary venturis, the more vacuum in the venturi the farther they open. If you rotate the throttle mechanism completely, only the primary butterflies will open until the vacuum is sufficient.
The Edelbrock carbs have mechanically operated secondary butterflies, when you rotate the throttle mechanism fully, all four butterflies will open. The amount of air drawn through the secondaries is controlled by an air valve above the secondary butterflies. On the Performer series carbs the air valve has a fixed counterweight and the opening rate is not easily adjustable. On the Thunder series carbs the air valve is spring controlled and is easily adjustable.