How does the overdrive work on a Ford Focus auto?

It makes the car have that "extra gear" which allows you to cruise the roads with a 4th gear rather than have only 3 gears without overdrive

THERE IS A BUTTON ON THE SHIFTER, UNDER THE "t" HANDLE. IT'S "PUSH ON- PUSH OFF" AND THERE IS A LIGHT ON THE DASH, BY THE ENGINE LIGHT THAT INDICATES WHETHER OR NOT IT'S ENGAGED. I WOULD RECOMMEND "OFF" FOR LOCAL DRIVING, IT KEEPS IT FROM UPSHIFTING AND DOWNSHIFTING AND DRIVING YOU NUTS.