Arghh... I know the physics behind this - but the name of the phenomenon eludes me. Maybe I've got anomic aphasia ... can somebody actually studying physics now tell me the name of this?
Anyway, contrary to what Protector of Mankind said above, it's not to do with heat capacity or thermal conductivity of coals or human flesh. The truth is that small droplets of water on the feet of the fire walkers, either from damp grass or their sweat, flash to steam at the outside of the droplets on the red hot surface, leaving a micro- layer of steam between the water and the heat. This thin layer of steam acts as a good insulator, slowing down heat transfer so that the water does not burn off for quite a while - many seconds.
It's the same thing as when you have a really hot frying pan and you drop a few drops of water on them. If the pan is hot enough, the drops of water will curl up into tiny balls - you'll be able to play with them by blowing them around the pan for a while. On the other hand, if the pan is not quite hot enough, the water does not flash into steam on contact and instead spreads over the surface of the pan and evaporates much faster. The surface in contact with the liquid must be much hotter than the liquid's boiling point.
Seemingly paradoxical but true. The thing is though, that if you are too confident about fire walking, you won't sweat ... and without that layer of water (or some handy damp grass), your feet are toast. So always respect the heat.
I learnt this secret eight years ago from a Physics textbook of mine. Still have not yet tried fire walking myself.
Anyone want to fire walk with me?
Sensei - sure. when, where?