It generates graphs that are doubly differentiable (function is continuous as are the first and second derivatives). There are two smoothing types: the first is a little more flowing while the second attempts to respect the initial data as local
I can write about how the math works in a future post if people are interested. These methods should not be used for interpolation for those interested in data mining or analysis. There are many other good algorithms for those uses.
And this is what the plots look like. The actual plotting is done by flot. Note that the data is generated randomly every time the page has reloaded. So I probably haven't seen the graph that you are currently looking at.
Update: I added the singly differentiable versions of this data. Looks pretty much like my previous plots. The type 1 singly differentiable plot is essentially catmull-rom to my understanding.
Update 2: This has been updated and blogged about here. You can find new examples there.