Well it is prob'ly some kind of archaic singing, if you stretch a point. Somewhere it was lamely called "throat singing". If they are using their throats, what am I using -- my medulla oblongata? Maybe we should start a new thread called Weird Singing.
I remember when I was a kid I found I could sort of sing, real low, by singing on the inhale, rather than the exhale. I wonder if it's something like that. I can't do it now, it sounds like I am croaking. Wonder if it would hurt my regular singing.
Also, do you know anything about "circular breathing"? I think you are supposed to be able to sing or play a continuous sound without stopping. Something to do with didgery-doo players, and a rumor that Raasan Roland Kirk could do this.