The dash button switches it from auto to manual mode. In manual mode, you use the floor switch to engage/disengage the overdrive. In auto mode, it is switched on/off based on the rpm signal from the speedo sensor. In either auto or manual, the control module switches the overdrive off below a certain mph and also prevents it from being engaged when you shift into reverse, which would cause bad damage. At least that's how the GV tech explained it to me.