I just watched a show on big constrictors on Animal planet. They addressed the issue of what actually kills the snakes prey. It seems to be a combination of 2 things. The snake can feel when the prey item exhailes and tightens its coils so air can not be drawn into the lungs. This is not the only thing that leads to death. The second I hadn't ever heard of before. The snake can detect the prey items heart beat and concentrates its strongest muscle group wich is near its head. It constricts so hard that it press's the sternum into the heart and stops the heart from pumping. This is not proven but speculation by scientist's. So the cause of death is asphyxiation by two methods.
On a side note the show said that 2 people die every year as a result of their "pet snakes" and they interviewed people who survived attacks and what they did to survive it.
very interesting show It is called "The Big Squeeze"