You set the power level with the controller display and the speed with the Nuvinci, so your Nuvinci gear-changer becomes your throttle. Set the controller to "Torque simulation" mode, which is current control. This'll make more sense when you try it.
The fact that the Nuvinci becomes the throttle, is only when used alongside a controller with Torque simulation mode, such as the S09P, or would it be the same with another, standard controller?
I ask because for us, center-Europeans, throttle is a forbidden fruit. So, torque sensors or eventually Nuvincis, which could act legally similar to a throttle, are ideas quite interesting to us.
I know I would know the answer if I tried, but I do not know anybody owning a Nuvinci, either N360 or N171 (which is very cheap
http://www.utahtrikes.com/PROD-11617994.html ).