So I’m about to snooze when all of a sudden before I drift off it feels like I missed a stair in my bed and I start to fall until I’m forcefully woken up by the shock. Like, WTH?! It’s not like a dream where you fall off a skyscraper, it’s more like before you start to dream when it happens.
It’s more nuanced than this but something along the lines of your brain having more “awake” activity than your body, so your brain interprets the lack of response from your muscles as a possible sign that you’re falling.