I'm a singer...
I've had lessons with a Jazz teacher about singing properly, never got around to the theory of getting high notes and wailing like LaBrie / Dickinson etc, but aboput growling and screaming (Lamb of God etc).... definitely dig deep from your gut. I actually have no idea how I'm doing it, but I do "feel" like it's coming from the diaphragm, and the actual raspyness is coming from several places. I try to use as much of my "piping" as possible, as making all the effort from your throat alone will mess you up. Jesse from Killswitch Engage (he was their first singer) actually BROKE his voice by screaming so hard. I believe this is because it was all created in his throat.
Even when screaming I try and produce it also as a "head voice" where you are projecting your sound from the diaphragm and if you can imagine it like a laser pointer, aiming the flow up your system and through the roof of your mouth, through the top of your head rather than concentrating it on coming out of your mouth. It's hard to explain without being face to face, but my jazz teacher made me understand it really well.
It's definitely a good idea to talk to a classical / operatic / Jazz vocal coach just to get at least the theory down, because a lot of it applies to screaming too in my book.
That's the "science" part, but now for my own personal input. FEEL it.
I'm talking dig deep in your mind, bring out your most primal beast. If you have to think of something that pisses you off beyond all reason then do it. There's nothing worse than someone trying to scream if you can't "feel" it. really build up a rage and then just let it loose in a maelstrom of audible anger. That's the secret ingredient. The science is nothing without the soul.
