I've had episodes of this since I was around 18. I didn't have the internet to help me figure out what was going on and no one I asked knew what I was talking about.
It seems very real and it can be actually terrifying. The best way I can describe the actual initial feeling for me is to imagine your soul coming up through your spine, very slippery and out through the top of your head as you are falling backwards.. ack... I can almost make the feeling while I'm awake and it's unpleasant.
Try as you might you can't really move or cry out and people in my households knew to wake me up if I seemed to be struggling in my sleep. I've had a couple of episodes that were so odd and disturbing they've stuck with me. Fortunately they seem to have phased out as I've gotten older unless I'm overly tired.
Interesting to see it mentioned here
Daughter of 1776 American Revolutionists