Mechanical secondaries with a variable venturi feature. Good points, straight from Carter and not from a sales brochure.

Venturi: A Venturi is a system for speeding the flow of the fluid, by constricting it in a cone shaped tube. In the restriction the fluid must increase its velocity reducing its pressure and producing a partial vacuum. As the fluid leaves the constriction, its pressure increases back to the ambient or pipe level.

A venturi DOES NOT control the secondary operation. It aids it.

Thank you.