The Airbrush Valve Piston Pin is a vital component part of the airbrush trigger button's function to open the air valve for airflow though the airbrush. This pin is located under the trigger, next to the air valve assembly, and is visible underneath the airbrush when the air valve assembly is unscrewed and removed from it. When you push down on the trigger button it utilizes this pin placement to press open the airflow valve so that the air flows from the hose into the airbrush. This is a precision manufactured part specifically designed for the "Mist"ique Airbrushes, and made of a strong, durable metal alloy. These valve piston pins have a way of getting lost when taking an airbrush apart, so it's best to keep an extra on hand. Your airbrush will not function at all without it!