Depending on the type CVT you have - TAV type or Variator type. Here's a link to Kid-me's you tube explaination for the variator type changing the sliders:
http://www.youtube.com/watch?v=-DadACJjrz0
If you are using the TAV type I can try to explain-
Clutch at the engine crank starts out small and as rpm's increase the clutch closes in on the belt and drives the belt upward to increase the ratio.Reason for this is that as the motors rpm's increase it can overcome the load of a larger ratio without bogging down. As the front pulley increases in ratio the driven unit decreases in ratio by driving the belt downward toward the bottom of the shaft to produce more speed less torque.Same principal as your axle sprocket-small sprocket ="higher topend speed with less torque", larger axle sprocket = "slower topend speed increased torque and climbing ability". Does this make sense?