If you practice paying very close attention when falling asleep (or napping), you may notice that as you cross over the threshold, you can see around you, as if with eyes open, but often if you try to focus too strongly on that, you will wake up. There is almost a peripheral style of awareness required, and by this I mean to allow yourself to see, but not try to see too strongly. Often if you try and focus on what you are seeing, it will dissipate, and you will "snap out of it" so to speak, so the trick is to remain aware of it, but not "look at it directly".
I had discovered this many years ago, as something I experienced when having naps, specifically as I was slowly falling in towards sleep. Over the years I have had many conversations with people who have also experienced similar things. The quality of the visual phenomenon is slightly different than seeing with eyes open when you are awake. I believe it is still information that is ultimately being fed to the visual centers in the brain, but it is not coming down from the rods and cones in the eyes, nor along the main optic nerve.
The stream of information is delicate, in the sense that if you engage some other mental processes (too strong of trying to pay attention to the visuals), then I believe it tries to access information via the optic nerve, and thus it severs the other connection.
There are, I believe, some similarities that can be found within the general practices of visualization. One can force to visualize, in their center of vision, or in the main field of their vision, and this may take a long time of practice to get good at it. The other type of visualization occurs without this direct focus, it is much more of a release, and the visual field in which this occurs can take place in the entire 360 degree sphere of vision, but it exists in a constant peripheral experience, which is to say, you are never "directly looking at it through your main conscious center of focus", but experiencing it as something that envelops you into it. For me, this style of vision is always a bit "off to the side", but I don't mean this in a directional sense, but a conscious one.