As I recall, it was a Native-American way of cooking in general. I don’t know if it were to be able to cook and transport food at the same time, or what have you.
What I do remember is the testing and learning which stones would be appropriate. The rocks would have to be dry, not from the river. If they were from the river, they’d be wet and if they were wet, they’d explode in the fire.
The idea was to heat the stones in the fire and then put them into a cooking vessel of some sort, (skins?), and the hot stones would heat the liquid and cook the food.
I’ll look it up and if I find anything I’ll come back… .