What are Hibernated tabs?

Hi

What is tab hibernation exactly?

The home screen's screenshot just says to use it if the tab is taking up too many system resources, but nothing else.

The tooltip says "When enabled, each time you restart Chrome, Sidewise will show any tabs you previously had open under a hibernated 'Last Session' row at the top of the Pages tree. From there you can selectively wake any or all of your previously open tabs."

But I don't see any Last Session row, just a 'Windows' tree. Or above that, Pages and Recently Closed buttons.

(In fact is there any documentation on how to use this app fully? There doesn't appear to be an overview of all the features or how to use them, and I'm a FF Tree Style Tab user!)