When the solenoid is advanced, it holds the throttle open farther for a higher rpm. The hex can be adjusted by turning the hex (screw it in or out). Mine is controlled by the NSS. When the car is in P or N, the solenoid is retracted and the idle is controlled by the carb idle screw. When the car is in R, D, 2, or 1, the solenoid is extended. Setup this way allows me to have a higher idle rpm in the drive gears, and then when put in P or N, I have those rpms lower which helps it shut off without run-on.