Iirc, the button is to shut off the overdrive (4th gear) for passing on the highway and what not, like shifting from 5th (overdrive) to 4th when passing on the highway with a manual transmission.
You should leave it alone for the most part.
Edit, just to clarify.
When you start the car, overdrive is on by default.
Meaning it will shift into 4th (overdrive) gear while cruising, lowering the engine rpm and getting better fuel mileage.
When you shut overdrive off (overdrive icon illuminated) it will not shift into 4th (overdrive) your rpm will remain high, and you'll get horrible fuel mileage.
Shutting the overdrive off (overdrive icon illuminated) is only for when extra acceleration is needed.
Someone please correct me if I'm wrong, its been years since my auto to manual conversion.