Your trip odometer tells how far you have driven since you last reset it.

Your trip/select reset switch will go back and forth between the odometer and the trip odometer if the button is pressed and released within 1.5 seconds. If the button is pressed and held for longer than 1.5 seconds while in the trip odometer mode, it will be reset to zero. If the button is pressed and held for longer than 1.5 seconds while in the odometer mode, it will have no effect.

Your trip odometer will work no matter what position your key is in, and even if the key isn't in.