回复：correct me if I am wrong

I have to correct it because:
It is correct that OD OFF allows your car to switch from A higher gear to lower one"S", in order to generate more torque ---That's why we need a switch to control on and off!
OD on does not mean that the transmission will automatically "add" one highest gear in addition to the lower other three gears, rather, OD ON will fix the gear on the highest gear. Therefore, during local drving, you are using the highest gear whenever you stop or start frequently, thus making your transmission wear out faster than normal. Just imagin why then will we need the function of OD on or off, if OD on can only bring/Add benefits/flexibilities, to the other three lower ones!
Be careful on this point.
Many recent cars, except for sport cars, do not have OD function anymore, because automatic transmission technology advances and consumer preferences, that is, the computer is able to adjust (or, the manufacturers "make the decision for you" ) as when to switch to the highest gear.