Incidentally, it’s also a demonstration that you shouldn’t use high contrast in typography. When you start the test you can clearly see the lines of text retained on your retina.
The same with MacBooks in dark mode, once you turn around you can see large horizontal lines separated at regular intervals that are maybe due to the refresh rate of the screen (or something else, if someone knows)
In my experience these are after exposures from lines of text. They get blurred together into indistinct lines because your eye focus moves between words, superimposing them.