The simple answer is, when the control cable is pushed in the overdrive can be engaged. While it is possible to push the cable in while the car is moving, never pull the cable out when the car is is in motion. The owner's manual states that you should not operate the cable at all with the car in motion but you can push it in when the car is moving. When the cable is in, the car will "free wheel" - there is no engine braking. Due to the way the over-running clutch mechanism works within the overdrive the car will free wheel any time the cable is pushed in (and you can leave it in all the time) but when the cable is pushed in you must put the car in reverse when you park it! If you don't, it can roll away if your parking brake is not set or not working. I'll restate, with the cable in, you must put the car in reverse when parked!
To operate the overdrive, when you reach 30 mph, momentarily lift your foot from the throttle and you should feel the car go into overdrive (feels like you just shifted a gear). When you slow to below 23 mph the overdrive drops out.
There is a "kick down" feature which allows you to return to conventional drive if you need to climb a hill or pass a slow moving vehicle. To operate the kick down feature, floor the throttle and you'll feel the car shift back down. Don't operate the kick down feature at speeds above 50 mph.
