When you release the trigger on your spray gun the pump does not stop.  The water being produced by the positive displacement pump has to go somewhere.  Without an unloader valve pressure would build up to a dangerous level and something would have to give.  

The unloader prevents a dangerous build up of pressure.  When the trigger gun is closed, the unloader valve regulates the pressure by redirecting the water that the  pump is producing back to the low pressure side of the pump.  In other words the water is re-circulated around the pump. 

This re-circulation is what can cause heat build-up of the water.  The horsepower being transmitted to the pump from the motor has to go somewhere.  Moving parts cause friction, which turns to heat. Without cold water flowing through the pump this heat continues to build up until serious pump damage occurs.  Most pumps can tolerate temperatures up to 140 degrees F.  Hoses, plungers, seals and packings can and will break down at these temperatures.  Many pumps are fitted with a thermal relief valve which opens when the water exceeds 140 degrees.  This dumps hot water, usually to the ground, which is replaced by cold water from the source.  While this can save a pump, it is no substitute for proper operating procedures. 

The unloader valve also acts as a pressure and flow regulator.  Most can be adjusted to bypass water from the pressure side to the low pressure side, resulting in less water being pushed through the nozzle.  Less water = less pressure.  This is not the recommended way of reducing pressure because it also reduces water flow.  Plus, it is not very efficient.  Your motor is working to make pressure and your unloader is bleeding off the pressure. Kind of a waste!  To reduce pressure it is almost always better to change nozzles to a larger size.  This way you get lower pressure with the same amount of water flow.  Same water being pushed through a larger hole =  lower pressure.

There are two types of unloader valves:

1. Trapped Pressure unloader valves are the most common of all, being found on most off-the-shelf pressure washers.  They are commonly used because they lend themselves to being able to regulate pressure.  Regardless of what nozzle is used, the trapped pressure unloader will adjust to the orifice size.  This type of unloader works by responding to the spike in pressure when the trigger on the gun is closed.  This spike results in pressure being trapped in the hose.  The unloader then bypasses water to the low pressure side of the pump.  Negative aspects of the trapped pressure unloader are the fact that the system from the outlet of the unloader to the shutoff mechanism of the trigger gun is under constant pressure.  Operators need to be careful when the trigger is pulled the kickback effect does not result in loss of control or injury.

2..  Flow Activated unloader valves work on a different principal.  These valves sense the flow and any decrease in flow initiates the bypass.  Flow unloaders are also called Soft Unloaders in that they do not cause a kickback as no pressure is trapped in the hose.  When the trigger is released the build up of pressure is gradual allowing the operator to prepare.  Flow unloaders are better for the system as there is no hammering on and off when the trigger is pulled and released.  As flow unloaders work by detecting changes in flow, within a range,  an operator can not adjust pressure by changing nozzles.  If a smaller nozzle is used or a piece of debris partially blocks the nozzle orifice a flow unloader will continuously cycle until the debris is cleared or the proper nozzle is installed