Adjustable Bleed Valve
The adjustable air bleed can be used to adjust the pressure to your airbrush where the compressor does not have a regulator. It can also be used where the compressor does have a regulator but you need to bleed off more air to reduce the air pressure lower than the regulator will allow.
For example, for airbrushing makeup, you should be able to regulate the air pressure on the compressor down to around 6-8 psi. When you come to airbrush around the eyes the adjustable bleed will allow you to drop the pressure to the necessary 3-4psi.