On the 76 model, it uses an input from the change switch to indicate if it's in or out of gear. When it's in gear, the solenoid is energized and allows vacuum to pass through the solenoid and apply vacuum to the servo that raises the idle. On the 77/78 models, it works the same except there's an input from the speedometer so that the vacuum is only applied when going slow (below 15 mph, I think). That helps to keep the high idle from fighting with the brakes when you try to slow down from high speeds.
....Paul
