if u take of the second gear u will see that there is a little wedge in the gear going in closer to the center of the gear.[img][/img]

As the engine gains speed the gears turn faster and faster .

as this happens the force pulls out a little bit of metal (not on the gear ... what the gear is attached to) which attaches and hooks onto the second gear where the wedge is. the car will now being running in second gear. as the engine slows down the force isn't as big and the spring has the bigger force than the engine and the little metal bits moves back into its original place which puts the car back into 1st gear.

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