Excellent point and I didn't know that. I saw the mechanical linkage between primary and secondaries and assumed the Carter/Edelbrock design was fully mechanical. This article explains it well: www.hotrod.com/how-to/engine/1306-vacuum-or-mechanical-secondary-carburetor/
To summarize, the generic term is "controlled-secondary". Holley uses a large vacuum diaphragm actuator. Edelbrock uses an "air valve" or "auxiliary throttle valve", which is actuated by the air flow and vacuum. Quadrajet and Thermoquad are similar. The mechanical linkage does open the secondary throttle plates, but that just "enables" the air door above that automatically restricts air flow thru the secondaries to what is needed. There are carbs w/ fully mechanical secondaries (Holley makes some). For those, the driver should not "floor it" at low rpm or the secondaries will go wide open and give poorer performance than if they modulate the pedal properly.
More sophisticated is a "variable venturi" design. Modern motorcycles that are still carbureted (ex. Ninja 250 in U.S.) have "sliders" for that. The Predator was an after-market one for cars. Ford used a variable venturi in early 1990's large cars which worked well but proved problematic.
