When the overdrive control cable is pushed in, the car can operate in overdrive. When you reach 29 MPH, let your foot off the throttle momentarily and the car will go into overdrive (like shifting to another gear). If you need a boost of power, you can mash the pedal to the floor and kick it down out of overdrive... then let your foot off the pedal again to go back into OD.
The thing to remember, and this is important, is when that overdrive cable is in, the car will free-wheel - the wheels turn independant of the engine so you need to put the car in reverse when you park it. If not, it can roll away.