Just as you mentioned, this switch lets me know, I'm gonna be asleep in the next few seconds. I also often listen to audiobooks or ASMR while lying down to sleep, and just as this state kicks in, I know its time to flick out my earbuds just before I lose consciousness. However, sometimes I'm even able to ride this wave right into sleep -- as in, from weird observer state right into the dream world. I was actually thinking about this a few days ago, and did some very quick searching, but couldn't find anything -- I wonder if we are observing the switching of our own brainwaves from alpha to theta?
I'd like to ask other people that experience this: do you also have high rates of sleep paralysis? I get it almost nightly. My dreams are also very vivid, and I remember about 80% of them. Also, if I wake up during a dream, upon falling back asleep, I'm able to "continue" them. Maybe I'll try this Dali method a bit, who knows!