The AC Compressor is driven by a belt attached to the engine. The belt always turns the AC pulley, but unless the compressor clutch is engaged by turning on the AC in the car, the pulley will not turn the internal parts of the compressor.
source/image(PrtSc): Gary’s Tradition Auto
When the clutch is engaged, and the internal compressor parts start turning, the pistons inside the compressor suck the refrigerant our of the evaporator, and force it into the condensor. Watch the video from Gary’s Tradition Auto:
The AC compressor is the power unit of the air-conditioning system that puts the refrigerant under high pressure before it pumps it into the condenser where it changes from a gas to a liquid.
The pumping action of the compressor will increase the refrigerant pressure from about 30 psi to about 250 psi. Compressors can fail as a result of weak internal valves, worn seals on the pistons, failed clutches, or leaking seals./Gary’s Tradition Auto