This technique covered over at DIYphotography is one I’ve been exploring a lot lately. It’s actually how I set up this shot, from about a year ago:
It’s fun, but hard to do well with speedlights. I’d recommend either continuous lights or something with a modeling lamp in it if you want to explore this idea further. It helps you see what you’re doing right away, instead of having to chimp in closer and closer with each picture.
DIYphotography did a really good job explaining it and showing all the possible effects, so I’m just going to leave it to them… check out the whole article, it’s pretty great.