How did she do this trick?

http://www.youtube.com/watch?v=zGhjtQx6O-g&feature=related

If you watch this video of Britney Spears being cut in 1/2 during her circus tour and have any idea how they do that, share the information. I know it is an illusion but I don’t see how she could fit in the top portion of it and I don’t know how her head is still there if she ducked down to the bottom portion. Maybe the face is fake or something?