The two types of unloader valve we carry are "Trapped Pressure" valves (valves that sense the buildup of pressure when the trigger is released and trap it in the hose), and "Flow" valves (valves that sense the lessening of water flow between the valve and trigger gun).  Both types will cycle the water back to the inlet in what's called a "bypass loop", as the water will continue to circulate until the trigger on the spray gun is pulled.  A Trapped Pressure valve has a noticeable kickback when the trigger on the spray gun is pulled, so handle with care.  However, you're able to freely change nozzle sizes when the pump is running with this type of unloader.  A Flow valve has no kickback when the trigger is pulled, but with this nozzle you can't reduce the pressure by changing nozzles while the pump is running; the unloader will sense the drop in pressure and begin circulating the water in a bypass loop.

Try not to let your bypass loop run for more than 2-3 minutes, because no new water is being introduced to the pump to allow it to cool down.  Recirculated water is a great heat conductor and will damage your pump when it gets too hot.