its very common in new zealand, over here we call them SPOTS.
its easier with two people, one to 'Drive' and one to take the spot.

press the spot carefully but firmly and quite quickly, and the spot will stick to the knife, then turn it over so you can see it, and push the other knife ontop,

the spot will instantly erupt into a cloud of smoke, and after a few seconds will be gone except for a bit of black residue on the knife.

DO NOT USE good silverware, only cheap/stolen/spare butterknives as they will tarnish even after one spot.