It's meant to be a positive return to idle if for some reason the throttle cable hangs and the spring doesn't automatically return the carb to the idle position. The cable allows you to manually do so what the spring can't for whatever reason. I don't know when they started doing so but was in the early 80's when the CV carb came into play. My 80 FXWG has a single cable but my 84 FLHTC had the double.