When a beam of incoming microwaves hits the flat, mirror-like surface, it is reflected (A), but when it hits a bump on the surface, the radiation scatters (B). Placing the cloaking material on top of the bump "restores" the mirror, so that the beam is reflected as if it had hit a flat surface (C). Panels D-F show the same cloaking effect with difference wavelengths.
[Image courtesy of AAAS/Science]