I remember reading that when your in a dream where your falling, or any other situation where your getting killed/dying, its your body going into such a deep state of sleep that your organs start to "sleep" too (I'm aware that they dont actually sleep but medical details might take a while on here

). This is also why you get that funny sensation when you fall in a dream.
And apparently, your bodys automatic reaction to stop this is to just jolt you awake, which is why a lot of the time you'll actually just shoot up and find yourself awake sitting up in your bed or just suddenly open your eyes and awaken.
Leading onto the next point, where if you do hit the bottom or get killed in your dream, your organs have already started to shut down and your already dead
Of course how they tested that one is beyond me
Scientist: "Right I think hes finished dreaming, hey buddy, did you get killed in your dream?"
Test Patient: "......."
Scientist: "Bugger, I guess he did, Oh well theorys correct then"