1971 - DISTRIBUTOR SOLENOID All 383 and 440 engines (except the 440 3-2V) have a solenoid incorporated in the distributor vacuum advance mechanism to retard the ignition timing when the throttle is closed. At closed throttle, electrical contacts on the carburetor throttle stop and with idle adjusting screw in the closed position, causes the dis- tributor solenoid to energize. This retards the ignition timing to provide reduced exhaust emissions under hot idle conditions. Cold or part throttle starting is not penalized because the distributor solenoid is not energized unless the hot idle adjusting screw is against the throttle stop contact. Timing must be set at closed
throttle to give ignition full retard.


Read all about it here

https://www.forbbodiesonly.com/moparforum/threads/vacuum-advance-w-solenoid.171617/