First off it demands a ridiculous amount of support (or energy), and then you basically let out air while making a "note" in a so dark sound colour it almost sounds evil (according to my teacher). :)

It's kind of hard to explain, but that's the main things, I guess. And it's a kind of sound that you never thought you'd like, but when you can actually do it yourself it's the coolest thing in the world. :)