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Google "Comet CVT". Same thing as snowmobiles and scooters.
The front (drive) clutch is centrifugally operated with a set of weights. More RPM gives more pressure to the sides, causing the belt to move to a larger circumference on the drive clutch (pulling it to a smaller circumference on the driven clutch), deceasing the ratio (increasing the output speed).
The rear (driven) clutch is torque actuated with ramps. More torque applied causes the pulley sides to squeeze together, causing the belt to move to a larger circumference on the driven clutch (smaller on the drive clutch), increasing the ratio (decreasing the output speed). A spring keeps the driven pulley sides initially closed together.
So you take off gradually. The drive clutch speeds up enough to initially grab the belt and the vehicle starts moving. As the engine continues to increase in speed, the weights cause the ratio to decrease. However, if you "floor it" or encounter a hill at WOT (etc.), the driven clutch forces a higher ratio and revs increase for the increased power demand.
Tuning these things is generally a balance between weight size in the driven clutch and spring weight in the driven clutch. Smaller weights/more spring pressure = higher RPM at max power.
Just rent/borrow a scooter and go for an hour ride. It will become clear pretty quickly.
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