there has been a recent article in some newspaper/website that does a pretty good job of explaning it. I thin RJ had a link to it on his twitter feed, as well as a topic here in the forum.
You can also go back and search for some threads in which Chris Andrews on this website had some actual power rankings he used to help set lines, as well as additional info on the hows, whats, etc. Couple of years back.
Start by searching some stuff from RJ. That article was in the last handful of days