[ignifero]

There's a better article in the guardian http://www.guardian.co.uk/science/blog/2011/jul/13/harry-pot... . They have an interesting intuitive description:

He used one set of lenses to prise open a gap in a beam of light, by slowing down long wavelengths, such as red, and speeding up short wavelengths, such as blue. With a second set of lenses, he then closed the gap, so at the end of the experiment, the light beam looked exactly as it did at the start.

I'm not sure that i understand it correctly, the "cloak" occurs because there is no light during the event as it has either moved on or is lagging behind. It's the recombination at the second lens that does the trick, giving a false sense of continuity to the observer after the lens.