A demonstration of Schlieren photography, allowing us to see air that is moving (youtube.com)

I wasn't sure what to expect, but wow this was really really cool.

I have a reasonable understanding of optics but I'm still trying to understand how this works. I'm looking at this explanation but still trying to break it down:

http://www.ian.org/Schlieren/HowTo.html

Anyone have a good intuitive explanation?