The speed changer is a varaiable diameter pulley. The inner shieve of the pulley is floating, in other word it can move side-to-side. The floating inner part and the two vee belts plus it being mounted on a movable spindle makes it all work nicely. When the spindle moves forward, the engine (front) belt runs higher in it's grove (larger diameter because the grove becomes more narrow) while the transmission (rear) belt runs lower in it's groove (smaller diameter because the grove is wider) thereby giving us more gear reduction. When the spindle is moved rearward the engine belt runs lower in it's grove while the transmission belt runs higher in its grove and gives us less gear reduction.