Whilst the nail polish is wet, you simply hold the magnet [which is on the cap and detatchable - very handy!] over the nail and wait for the effect to develop. You end up with a lovely wavey effect on your nails. I've seen you can get other magnets to produce different patterns as well.
Now I'm after more of these nail polishes, and I may just have to nip into boots when in town later :)