For those who didn't take Science 10 in high school, the concept works by photoreceptor cells in your retinas losing sensitivity after being overstimulated -- so if you flash 'em and then close your eyes (or look at any blank surface, like a wall or piece of paper), the cells corresponding to the flashed image will send out weak signals to your brain in comparison to the unflashed cells, which signal normally. Result? A reverse-color "cutout" of the letters B-M-W hovering in your visual field.
This post would be utterly pointless without a video, so here you go:
In summary, I tip my hat to BMW for thinking outside the proverbial "advertising box" in which most companies find themselves restricted. There doesn't really seem to be too many major breakthroughs in advertising these days. However, seeing the letters B.M.W engraved in my retinas for two seconds isn't going to make me shell out $40,000 for a 3-series if I didn't already want one.