Did it ever happened to you when you are relaxed and kind of falling asleep, you suddenly start shaking your legs or whole body and wake up? Well, the term for this is called Hypnotic jerk:
I have had that before. It has happened when I was falling asleep and also deep asleep dreaming of falling. It is interesting that they tied the two of them together in this study. It has been happen less since I have gotten older, slowing at about 40 and almost gone at 50 plus.
I get this the most when I'm falling to sleep on myself and suddenly I just jump up. I figured it has to do with the last conscious thing I was doing and the subconscious says to my conscious wait you're supposed to be doing... Then you suddenly get up, Thanks for the technical term though I will have to remember that - hypnic jerks, sounds like if you fall asleep on yourself you're a jerk .
I think this may be related to hypnogogia, hallucinations that occur when waking up. If you're dreaming in this state, I'd say what you're seeing is being treated as real and personal by your own perceptions, so you actually react physically as if receiving physical stimulus - I'd say the curious part is why it is pretty much always the same state, the falling, that causes this - it is similar to the hypnogogic sleep paralysis state, where it is common to feel "evil" around you, see figures, feel like you're being watched, but be unable to move, etc.