How do I implement an infinite scrolling grid on ext js 4.2.2

Hello,

I'm trying to implement an infinite scrolling grid on ext js 4.2.2. It seems like the documentation on this is kind of incomplete / confusing. I've set buffered: true on my store, gave it a pageSize of 100, added a Buffered Renderer Plugin to my grid, and made sure to set numFromEdge to 20.

However when I scroll to the bottom, nothing happens.

On the backend I don't see any ajax requests firing. What else do I need to do on 4.2.2 to get this to work?