It took me a while to figure this cast on out. Here is the original posting in photos on knitty. I found it easier to understand this cast on visually so I put a short video together with my interpretation of the cast on.
This cast on was originally designed to create a cast on that worked in both directions for the top of toe up socks. It can also be used anywhere you need a provisional cast on to create live stitches to work in both directions.