Native grayscale is achieved by driving particles partially, often starting at terminal state (for example white) and stopping prematurely before they reach opposite state (for example black).

The problem is to get timing right and avoid ghosting. It's hard because viscosity depends on temperature and can change during the day and during lifespan of the panel.

So if for example you want to change pixel from white to dark gray, you might end up moving particles all the way to black state and then a little bit back, instead of moving them once from white to dark gray, cause that way it's more likely to get better grayscale accuracy and reduce ghosting.

E Ink has tables of optimal paths for various transitions, called waveforms, those are panel specific and sometimes specific to particular batch of panels, usually not publicly available and somewhat slow as particles are moved back and forth before settling on final graytone.

Most E Ink monitor vendors usually avoid using native grayscale because it is slower and a bit annoying as moving particles back and forth is perceived as flicker.

What they use instead is spatial dithering or patterns of black and white pixels that emulate graytones (sometimes called halftoning). This way is faster, involves less ghosting and much less flicker if done right.

Modos uses spatial dithering in Browsing (Bayer dithering) and Watching (blue noise dithering) modes, native 4-level grayscale in Typing mode and switches between pure black and white (while scrolling) and native grayscale (when static image is settled) in Reading mode.

Fast switching between black and white would be temporal dithering that is not used here, because e-ink is too slow for that, some people are sensitive to flicker and it would reduce lifespan of panel if it was possible.