warmth meditation?

i cant remember what it was called but i saw a documentary and it showed monks that could apparantly warm their bodies up with meditation. and not just feel like it, acctualy warm themself up. they sat in a cold room and covered themself in a cold wet sheet. and they meditated and they sheets started steaming and dried and the monks warmed up. apparently it would have coursed severe hypothermia and maybe killed most people.
they say they did it by vissualising/imagining they are breathing in fire.

does anyone now how to do this? in more detail.

i am interested to see if there is other meditations that can do things like this?

Yeah, I have heard of this before. I was actually watching something on Discovery channel about a guy who could do the same thing. They actually measured his body temperature to see how much he was able to raise it and how quickly. Then they put him in hot sauna to see if he could also lower his body temp, something he had never attempted before. He was able to lower his body temp at will as well.