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The needle blocks the air and paint passages at different places inside the gun, so you can pull the trigger about 1/4 of its travel and no paint will flow out, just air. This allows you to adjust the air pressure before you start painting. Most HVLP guns work best with 27 psi entering the gun. By the time it reaches the tip, air pressure will have dropped to just 10 psi, thus the low pressure in HVLP.