[Tommy]

Generally speaking, vacuum secondaries are controlled by the driver's right foot but moderated by sensed engine vacuum. This prevents the secondary throttle plates from opening fully until the engine vacuum signals it is ready for all that air. ... Mechanical secondaries are controlled entirely by the driver's right foot. If the driver shoves the throttle to the floor, the primary and secondary throttles open fully immediately. That means it is up to the driver to modulate the application of throtlle so he can achieve wide open throttle as quickly as possible without creating an imbalance in the air/fuel ratio. For all these reasons, vacuum secondaries are generally preferred for street use while mechanical secondaries are preferred by seasoned racers.