The TQ or Thermoquad operates the metering rods principally mechanically, then as the secondaries are opened, by vacuum.
The step up piston uses a lift rod that extends down through the fuel bowl into the throttle body. The end of the rod in the throttle body rides on a lever, which is operated by a cam attached to the primary throttle shaft. As the primary throttle valves are opened, the cam forces the lift rod up, pulling the metering rods out of the main jets. This allows more fuel to enter the main circuit.
At high throttle and as the secondary valves are opened vacuum takes over and pulls the lift rod even more out of the main jets.