How to do the Glitch

If you are flying with elytra, if you hit a block you get into a strange phasing state that allows you to do this glitch

After you do the Glitch

You are now able to go through blocks that would usually block you at head level (keep in mind if it's also blocking your bottom half you won't go through), you can go through blocks that don't take up a full block (eg. stairs, trapdoors, etc.), and if you jump in an area that's two blocks tall, rather than doing a head hitter, you go right through the block. But, if you do the last one but half of where you are jumping into is air, you start x-raying and can see multiple openings if you are underground. This is also way easier to do if you replace the block right above you with a half slab that makes the height of the area 2.5 blocks tall (this also works in version 1.16.201 and I cannot provide photos or videos as I play on an Xbox and I am writing this on a computer (MacBook to be specific)