Smacking your head on the back of the jump happens when you are all butter and no pop. If you lean way back, when your tails release you have zero control over the rate of your flip. Best advice I ever got was fuck the sky. Throw your hips up to the sky. Other good motto is, "Look for the ground; you'll get around. Look at the sky; you're going to die"!
When you extreme tail butter a backflip, you will not be able to stop your rotation and you are likely to do a one and a half even if you don't smack your head. It's just not a good way to do a backflip. Remember to pop and look for your landing and life will be good.
I recommend trying it somewhere where you have a powder landing first. If you don't have powder; break up the landing 12 to 18 inches deep so that if you over rotate, you won't wreck yourself too bad. Also, your pop should be the same as if you were doing it on a tramp.