Disney's team uses an algorithm to create the different feelings. There are three different parts to the algorithm which Disney explains like this: "1) Calculate the gradient of the virtual surface we want to render, 2) Determine the “dot product” of the gradient of the virtual surface and velocity of the sliding finger, and 3) Map the dot-product to the voltage using the psychophysical relationship." Well, of course it sounds simple when they say it! The algorithm is based on the discovery that when our finger feels a bump when we slide over one, it is because friction compresses and stretches the skin on that finger.
There is a whole paper that supposedly proves that this works and we can tell you that it wasn't written by Goofy! Obviously it is still early days for this kind of technology, but eventually this could lead to some interesting things down the road that you might consider kind of cool.
source: DisneyResearch via Gizmodo