It's default on, when u push the button it will show "O/D OFF" and your rev go higher.

So leave it ON for normal driving, OFF it if you need that extra bit of power to overtake.
I do OFF it instead of pushing to 2nd gear/brake to slow the car a little sometimes

OD is not NOS or whatever....its not "geng" =.=" ...when u press that OD button ....what it does is sort of downshift 1 gear and ur revs will go higher so u will have better acceleration and when u press the OD button means it is OD OFF