The flash kind of "resets" each pixel. You can update without the flash (and some e-readers support this) but artifacts slowly accumulate. In the video they s

The flash kind of "resets" each pixel. You can update without the flash (and some e-readers support this) but artifacts slowly accumulate. In the video they show a mode where it is B&W while scrolling, but after stopping the scroll it flashes and renders in gray-scale. The narrator mentions several possible modes; noting that 16-shade gray-scale is too slow for any interactive use (including typing), but 4-shade can allow typing and B&W can allow fast scrolling &c.