The place where I first read about it is here: http://math.ucr.edu/home/baez/physic...Light/FTL.html

It doesn't really say, and I don't understand some of what they're talking about anyway. Whatever I tell you about the experiment would be my interpretation of that site, and a couple of others, so I guess it's best you look yourself, because I would probably garble some stuff up. It's under section 11 if you want to see. Not much on it, so you can see my knowledge of the experiment is quite limited, and probably distorted.