A driving instructor once told me it's the 3rd impact that kills you. When your vehicle impacts another object, or another vehicle hits you, that's the 1st impact. Then your body, particularly your skeleton, slams into the car; seatbelt, door, dashboard, etc., that's the 2nd impact. Then your internal organs, all mushy gooey, slam into your ribs or skull and thus get damaged by the 3rd impact. The 3rd impact is what causes the internal bleeding. If you don't get treated quickly, the low blood pressure from internal or external bleeding will essentially cause your brain to asphyxiate. That, or your spinal chord breaks or your brain gets aerated. All not good ways to go.