@Alborghetti: It has nothing to do with screen technology (and neither will the Kindle's own option).
Fire up your favorite image editing program, find a Contrast slider, play with the slider. That's pretty much the exact same thing.
You can do some slightly specialized trickery with a font rendering engine (e.g., FreeType's stem darkening feature), but the idea is basically the same: more darker pixels than what the standard AA grayscale rasterizer ends up with.
No-one's magically making eInk particles darker and/or the background whiter.
EDIT: CRe's implementation predates's FT stem-darkening feature (by, like, a lot
. And gamma correction on FT's buffers is actually always a good idea anyway), and appears to be implemented via a
variable gamma ramp LUT. So, quite literally making existing pixels darker, without actually affecting the outline rasterization directly.