I have always mentally separated what I described in my first comment from what I have come to associate with the term hypnic jerk.
The first is more of a mental, physically still thing. The other is, for me, a sudden motion of a large body part, usually linked to dreaming about falling off something. More importantly, the latter is emotionally neutral. I wake up going "oh, I didn't really fall. It was just a dream."
When the former happens, it is very emotionally uncomfortable. It's a feeling of pure dread, but sort of wrapped in cotton.
