We also had some good discussion on it on this thread -
http://kfclub.com/forum/index.php/topic,3737.msg21865.html#msg21865A couple of short answers - the car will operate in overdrive when you have the cable pushed in. Never try to pull the cable out when the car is in motion. The owner's manual says not to operate the cable at all when the vehicle is in motion but you can push it in when the car is moving, just do not pull it out when the car is moving.
Another important thing to remember - when the cable is in, the car will freewheel, meaning the wheels rotate independent of the engine. This means as long as you have the cable pushed in (and you can leave it in all of the time, I do) you must put the car in reverse when it is parked, otherwise it will roll away! Repeat after me - if the overdrive cable is pushed in, put the car in reverse when it is parked!
Good luck.