the explanation in wiki is correct. it means when u r driving ur myvi, the overdrive is ON, thus ur 4th gear is actually an overdrive gear (or virtual gear). so when u turn OFF O/D, u won't get the 4th gear for cruising instead you will be revving your lower gear for more torque to be used in situation like hill climb and stuff like tat. Conclusions: just leave it on if you are not sumone who suka rev kau kau.