So i am getting an 04 mazda 6S and i am curious about the overdrive feature. I have tried to find info about it on the web, but i can not find any details. I was wondering if anybody has any insights to using it or should i just keep it in drive the whole time.

 

5th gear and usually 4th gear in the manual and the highest gear in an auto are almost always OD's these days.
Any gear that is greater than one-to-one is an OD. There is no separate switch or feature to activate it... not like the old Volvo's or British cars has w/the Laycock OD.
That was actually a transmission behind the transmission and was solenoid (electric) activated by a switch inside the car.