I read a book, just a few weeks ago regarding this specific topic and my explanaition is directly from that book. We do have specific brain areas for this.

This sudden jerk you can read about it here: https://en.wikipedia.org/wiki/Hypnic_jerk and its not scienctificly clear why and how it works, the best assumption is that its a reflex.

You are not disconnecting your brain from anything.

Your brain disconnects: https://article.imrpress.com/bri/Landmark/articles/pdf/Landm... is probably the best paper I know of that describes the actual process going on in detail.