Jason Khalipa has a quick and cheap way to simulate a rings pulley system. Just use the straps that come with your rings, an step through the loops.Works best if you have your starps around a smooth bar. For a more advanced version check out the DIY ring training pulley system guide.
Carl Paoli teaches 2008 CrossFit Games Champion Jason Khalipa the backflip. A nice step by step break down into useful progressions.