The hidden switch is used to activate tests which verify proper operation of the fuel level, fuel economy, and oil pressure gauges and inputs to the cluster for engine RPMs and fuel level. It is all explained in the diagnostic manual for the instrument cluster.

Also, I seem to remember that the later clusters with the electronic odometer display has a facility for using the hidden switch for calibration of something...It is intuitive that it would be the speedometer/odometer, but I think it did in fact concern fuel consumption.