nuvincy is a continuous variation gearbox in the hub, for bicycles. Not for motorbikes. Don't try and put more than about 250W through a nuvinci, or it will break down quickly. Its principle is really simple but very clever made. The central sphere and the 2 sliding rings or so, connected to the sphere with a special fluid, which is both like oil and like glue, more or less, is a masterpiece of engineering.
Please keep in mind, for the sake of simplicity, that electric motors are sooooo good that they generally don't need any gearbox or variable transmission. Just a simple chain or belt or gear and it's done. One can say that the motor controller acts like a variable gearbox, changing the quasi-fixed battery voltage into any suitable motor voltage to get the desired torque at the given rotation speed. It even can reverse the rotation without any mechanical add-on.