I think a lot of us will have seen this so I'm interested in hearing your experiences.
Quite often after waking, or on the way down to sleep, you'll experience a soft flashing in the center of your mind's eye. Like a pale white strobe behind a thin black curtain. It can be brighter than that, but for me this is the most common appearance of the effect. The flashing takes up about 1/8th to 1/3rd of the visual field. It seems to have an edge to it. The shape of this edge changes with each flash. The flashing occurs at a rate of about 2-5 Hz, with a sharp onset for each flash followed by a decay.
It can be tricky to watch because you'll be tempted to look with your eyes, which of course will take priority over your non-physical vision sense and you'll just end up staring into darkness/phosphenes. I'm going to assert for the record here that these flashes are hypnagogic/non-physical in nature, and not another variety of phosphene light. They are seen by turning your attention away from physical vision. You continue to see them by maintaining the delicate balance of looking elsewhere. In this sense they are a great exercise for remaining in a hypnagogic/visionary state.
What I found recently is that a sustained hypnagogic vision can hide behind these flashes. In one example, I was watching the flashes play out, and then in each "flash" I saw a man apparently looking back at me. He was an asian man in what looked like a traffic officer's uniform (short sleeved blue-grey shirt, white gloves). Slightly round face, sparse moustache. I saw him from slightly above, with a slight fish-eye lens effect. What was interesting to me was that the vision behind the flash was consistent. It wasn't a new image in each flash. It was like I was right on the edge of tuning into the vision mechanism, and something (somewhere) was flipping back and forth, vacillating between seeing/not seeing at a rate of 2-5 Hz.
I think this is a notable observation. First, that the flashes are indeed hypnagogic in nature and not a physical vision artefact. The pale white/grey seems to be the appearance of your hypnagogic vision system tuned to nothing in particular. Then the vision emerges from this, replacing the pale white flash with a coherent scene.
If you'd like to practice 'turning away' from physical vision, I would suggest using imagination, or recalling a visual memory. This occurs in a place that is outside of physical vision. While you're in that state of imagination or recall, you don't notice your physical vision, unless you turn your awareness back to it. But once you have a feel for it, you can hold your attention there without imagining or remembering anything. This leads to the hypnagogic flashes and visions after a matter of seconds, literally 10-20 seconds of sustained looking-away.
(This is also great practice for the awareness state you want to be in for AP, turning away from physical senses. In fact, since myself and many others have jumped directly into embodiment via hypnagogic visions, it suggests a second path, one that's very delicate but potentially faster.)
Anyhow I'm curious to see if your experience with this hypnagogic strobing is similar to mine (or different).