Unintentional, uncontrollable movement.

Didn't quite know where to put this, since it doesn't really fit any categories.

When I'm trying to fall asleep, Or occasionally when I'm WILDing, my mind will sometimes wander off to a scene. I'll go with it and immerse myself in an increasingly realistic environment. And then, with no warning, when I make a sudden movement in my mind, I'll make it in real life and completely wake up.

For example, One night I decide to play tennis. After a while, I swung my arm in real life. This also happened a few nights ago when I got punched, and I real life I jerked back violently, about a foot or so.

Does this happen to anybody else, and whats up with it?

That's usually what happens when your body isn't yet fully immersed in the dream and Sleep Paralysis hasn't set in. This happened to me a lot in school when I would doze off and purposefully let my mind pick up some extremely realistic scene, only to suddenly jerk or move because I wasn't fully asleep.

Basically, because you haven't hit Sleep Paralysis fully, you could, at any time, send the actual signal to make your arm move, rather than thinking about it.