It was a fairly straight-forward process, really: the guy who was doing the light-writing was holding a strobe, which I triggered remotely (though it would also have worked for him to trigger it) while he was in action. The strobe was aimed at the wall, so it lit up the wall everywhere where he wasn't between the camera and the wall. That part of the wall stayed dark, with respect to the flash -- though there was enough other light on the wall, too, that during the rest of the exposure, as he moved around and eventually out of the frame entirely, the other light accumulated in that shadow, thus giving the mixture of visibility and darkness on that one part of the wall.
It's a technique I discovered from watching {tcb}'s flickr stream. He uses it to great effect in a number of his images, especially (in my opinion) in his Alien Abductions series, e.g. one of my favorites:
Thank you for the inspiration, {tcb}!
Everyone else: Give it a try. Happy shooting!
