I've experienced this intermittently my whole life. It is a weird feeling, you do try to catch yourself, but you get used to it I think.
caithiggs, you were saying it is related to sleep paralysis. I didn't realise that, but I suppose it does make sense. Your body is slipping from one state to another, and being quickly pulled back to awareness, jolting you.
Talking of sleep paralysis, has anyone ever experienced this will awake? It is horrible! I used to get this all the time growing up. I would wake up unable to move, but could see/feel people moving on the periphery of my vision, and it was very scary. I used to have to try to edge my hands to the edges of the bed (they always seemed to start out under my stomach or chest - I used to sleep on my front a lot); as soon as my fingers could grip the sides of the bed I was able to pull myself up and break the paralysis. I think this is wear you awake, but you are still dreaming, so you can't move, and can still see the things going on in your dream. It is a horrible experience, that i'm glad I no longer get.