And since Autometic Gear Boxes change gears when the ECU detects the right speed to do so, it was very hard to keep the balance between comfort and speed. A Auto Gear Box that'd shift hard and rev high for more power and soeed will have tranny thrashing noise at highway crusing speeds, the driver will have no control over the gear box when he wants to cruise at -lets say- 120km/h and the gear will be stuck in 4th not 5th to sooth the car down. Because the Gear box is following the agressive shift/rev utlization from the ECU for speed not comfort.
Likewise for a comfey gear box which will keep the gears high and the revs low will make 100HP feel like 50, and the HUGE V8 muscle machine with a auto tranny will go like a I4..
This little problem made way for this button. I am not sure -and no one else is either- which company launched it frist, but it just happened to popup in the late 80s in almost all automatic cars.
What the button does in Toyota's case is change the ECU's approch to shifting gears, hence the name 'over drive'. If your in the city, in a stop-go situation, slow or traffic infested drive condition keep the it to OFF. This will keep the gear box from shifting too soon, will not use the top gear and deliver more power. When you are ona highway or crusing at a constant speed just push the button to ON and the ECU will not use the higher revs, keep within the torque range and shift before time and the top gear will open up.
This difference is not noticable under normal conditions, but if your on a smooth highway, keep your car at a speed at which your secondly last gear (be it 4th or 3rd) ends and just press this button, you will feel it that the car lost revs, quieted down, and climbed a gear to top gear but kept the same speed. Your car will feel like it calmed down, gave up the struggle to deliver accleration and eased down to mentain the speed. This helps with long range economy and a better smoother ride aswell.