You actually use your diaphragm for all vocals especially ones using grit. What I reccomend is trying to gargle with no water. Feel where that sound is coming, it's from your soft pallette. Now imagine the sound that your voice makes when you first get up in the morning, that crackly grumbly sound. This is called vocal fry. Now you want a combination of these two effects and focusing the sound always at the soft pallette which is clearly above the throat and vocal cords. This is so you don't burn out your throat. Now before you do the growl breath deeply filling from your stomach up. Now focus the force downwards kinda like going to the toilet. This is how you do growls so they don't hurt you and properly.
Here is a clip of me trying to explain it, I am not a growler primarily at all. I sing power and traditional but this is something I tried learning.
http://media.putfile.com/How-To-Growl
I repeat I am not great