Underneath the sheets, John is using his custom templating library Radius, on top of which he’s implemented his so called Behaviors. To help you dig into such features, John has provided some starting points for learning Radiant as well as a quickstart for Radius. If you want to jump aboard and help the development, he’s providing some tips on how you can contribute.
svn co http://dev.radiantcms.org/svn/radiant/trunk/radiant/
Things called a CMS are notoriously complex. Cheers for keeping it simple.