Well it's like a normal stencil that you would use with spray paint except you use light (a camera flash) instead of paint. What you do is you set up a camera to take the photo with a long exposure (like 15 seconds). You set off this camera then run in to the shot and fire a flash from another camera (or wireless flash) behind the stencil through a pyramid shaped rig painted black and with tinfoil on the inside attached to the back of the stencil (which is also painted black) facing back towards the camera taking the photo. Once you've done that run back out of shot so you don't appear in the photo.
That's really awesome This light stencil thing is really interesting! I'd like to know more about it so I can try it myself when I get a new camera (mine was stolen). Is there anywhere I can go to read up about it?