The title basically says it all, but I am the most interested in what the rollers actually do?
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Summarizing my own answer to Engine braking with an automatic transmission:
Using the pictures from the Nissan CVT overview site:
You can see that there are two variable diameter pulleys connected by a very robust belt. This theoretically allows for an infinite number of gear ratios. Practically, there are plenty of engineering and human interface limitations that reduce infinity to a more manageable set of presets. There is a finite resolution of the pulley adjustment mechanism that allows the engine computer to direct the transmission. This means that, while not quite infinite, you probably have many many more effective gears settings than the five choices I have in my car.
The rollers allow you to continuously vary the diameter of the driving pulley. As the speed of the driven wheel depends on the ratio of the driving and driven wheel diameters, you can see that the rollers can provide try with a continually varying ratio.
Pretty certain we have a few questions on this topic here. Have a search around.