Either the good old A2 waveform update mode, or the 'new' GL4 they added somewhere in FW 5.5.x.
Which might be toggleable in some hidden menu somewhere, I guess? The other reagl stuff used to have one.
Note that I've never played with them in KOReader, because it's mostly terrible for text (as can be seen in the horribly aliased output in that video) [at least A2 is, can't speak for GL4, never tried it].
(cf.
https://github.com/koreader/koreader...-kindle.h#L114, which is commented to address the question of when stuff was added to the drivers)