It is an additional "long legged" gear, designed to allow the engine RPM to drop to a lower level while cruising at a fairly decent and constant speed.
However, the engine has less instantaneous power at the lower RPM, so the transmission has to "Kick down" for higher loads, like passing, big headwinds, steep hills, towing, etc.
In these conditions, or if you sense the transmission is kicking down too much, turn the overdrive off.
Otherwise leave it on and available for higher speeds.
pretty much when the overdrive is on then you will get more torque out of the engine cause it changes the shift points. in an SUV or Truck it can help in towing but in a car it is typically installed if you are ever driving up a hill. It will probably give you a slight edge in 0-60 but not anything drastic. it can have an effect on your engine if left on forever so if you had an engine rated for 100k miles, with the overdrive on it might shorten the life to 90k miles.
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