What does the orange indicator arrow Indicate?(84 rabbit)

[shaskill]

According to the manual, my 84 Rabbit should not have an indicator"idiot" light in the form of an off/on orange arrow. I assumed it was like the idiot light on my Chevy S10 and useless but when I started adjusting the fuel screw on my non turbo engine, as I decreased the fuel (less smoke), the light would start staying off at 55 mph whereas when I got the used car it was pretty much on all the time. Still is on in 3rd except without a load. Is it actually measuring something aside from RPM's? Vacuum?
I know it's a dumb question but....

[MayorDJQ]

It's the upshift light.  There are probably two switches on top of the IP, operated by the throttle rod.  These are what activate the light.

[shaskill]

so why is it sensitive to the fuel available from the pump?