[OtterBooks]

So far as I know it's purely ornamental, since the e-ink screen remains showing the last image it was instructed, kind of like an etch-a-sketch. An electrical signal directs the pixels (if that's the right term for them) to either display dark or not, then it uses little or no power. In fact I think you could tear the screen from the device and it would continue to show whatever image is on it, without power.

I may have gotten some details wrong, but point is it isn't a screensaver in the same sense as that term is usually used.