As you can see in the image below, these low-power screens (presumably using e-ink technology) should be just as large as the screens of the smartphones that they're accompanying. The screens might be useful in various scenarios: they could act as full QWERTY keyboards, display detailed notifications, help with composing emails (by showing a full calendar, for example), and so on.
Russian company YotaPhone already has smartphones with secondary e-ink displays (the latest being the YotaPhone 2), though its approach is different: the secondary screen is part of the phone itself, being embedded on the back.
Reportedly, Microsoft Research is "in deep talks" with the Lumia division to see if it can turn these flip covers with embedded displays into real products. Unfortunately, there's no way of telling when, or if that's actually going to happen. Until we hear more about this, let us know if you like Microsoft's idea, will you?
source: WM Power User