An unloader valve works physically, automatically or pneumatically using air pressure. When the compressor is indicting the system the inlet valve is open and the outlet valve is closed. If the discharge pressure from the compressor surpasses its safe limit 10 bar or 145 pounds per square inch (psi) -air force acting on a spring lifts a diaphragm or membrane against the downward pressure and opens the unloader valve.
1 Additional Answer
Unloader valves do not always work in a similar manner. Different manufacturers have different techniques of tripping their unloader valves but its purpose is to unload trapped air over a compressor piston. You can find out how different designs work from websites like ehow, epowerwash and youtube.