Typegenius "Sharpness"

Kobo's "Typegenius" feature (accessible if you select "Advanced" next to a font name) lets you adjust "Weight" and "Sharpness."

I get what "Weight" does -- it makes the characters bolder -- and can clearly see the difference as I adjust the slider.

But I don't understand what "Sharpness" does. I can't see a visible difference on my Kobo Glo (although I have aging eyes, so that shouldn't be taken to mean that there is no difference). What, precisely, does Sharpness adjust? What does it change on a per-pixel basis?

Thanks!