Turbos, throttle flaps, EGR valves, air heat exchangers, swirl valves – some of the many actuators who’s movement is controlled by a vacuum circuit.
You have to remember that the actual force is provided by atmospheric pressure- the vacuum circuit removes the air from one side of the diaphragm or piston of the actuator and atmospheric pressure pushes the other side, therefore there is a limit to the force that can be applied (about 14 pounds per square inch of diaphragm / piston area)
For now though we’ll assume that vacuum pulls on the actuator diaphragm.
The circuit comprises a vacuum pump, (usually driven by the camshaft) a vacuum reservoir, electrically operated solenoid valves, diaphragm actuators and small diameter pipes to link the components together.
In order to make a mechanical actuation, a computer sends a signal to the solenoid valve which opens the valve and allows vacuum to reach the actuator diaphragm and move the component in question. The position of the component can be infinitely controlled depending on the shape of the signal sent to the solenoid valve