Kinda weird the way Holley describes the secondaries. But then, they still say to choose your power valve by half your idle vac, so who knows?

Anyway, the secondaries are actuated mechanically, and the air door is opened by air velocity. Just like the AVSs and Thermoquads of old.
-----------------
" Full-Throttle Operation And The Secondary Air Valve;

The Street Demon has a sort of hybrid version of the two. The actual secondary throttle-plate is mechanically actuated, but progressive linkage begins to open the secondaries when the primaries are opened approximately 30 degrees. At this time, the secondaries then open progressively faster until reaching full-throttle at the same time as the primaries. However, instead of an additional accelerator pump, the Demon carburetor uses a spring-loaded air valve to control the transition.

The secondary air valve is nothing more than an aluminum flap that closely resembles a choke plate, which restricts airflow into the secondary venturi. As the secondary plate opens and air requirements increase, pressure drops in the venturi, causing atmospheric pressure to push the flap open. Spring pressure controls the amount of pressure differential that is required to open the valve. The result is a smooth secondary transition, regardless of how fast the throttle is opened."
-------------------
https://www.chevyhardcore.com/tech-stori...mon-carburetor/

The Street Demons are easily the best street carbs I have ever installed. Better throttle response with the least fiddling around to get them close. Most other Holleys are way fat on the cruise circuit. For example, the 3310 is usually more than 4 steps too rich for a big block Mopar.

Lots and lots of positive tests and reviews on the 'net if you want to search. About the only ones who dis the Street Demons are people who haven't tried one on the right engine.