Recently been working on a site with this stuff, until now this has been really poorly documented (you may have seen a page with a .mspx extension - that's an MNP page). Anyway, Tim Sneath mentioned an article which was recently pulbished which covers in far more detail than I'd seen before, the structure and implementation of this system...worth a read!
UPDATE: Just in case..here's the reason I can mention MNP - otherwise it'd have been covered by my NDA and I'd have had a pinky removed (that's Microsoft right, I keep confusing Micrsoft and the Yakuza)