Basically, no violent blow is allowed, and the animal must be
alive and all blood must be drained form the animal.
The problem is that of stunning the animal. Western abbatoirs will
frequently stun the animal so that it is unconscious before killing
it. Some muslim Abbatoirs forbid that, thinking that the blood
might not drain out properly. Nobody knows if the animal can feel
its throat being slit and its blood draining out, but clearly I
(and you) would prefer to be unconscious would you not?
There has been some noises made that it is not against halal
practice to pre-stun the animal but let's face it, Islam is a
primitive religion (compartively) and old practices die hard.