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 work

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 http://www.ian.org/Schlieren/HowTo.html Anyone have a good intuitive explanation?