If you're working on an important document in a busy environment and don't want people to see what you're doing at a glance, then you could use a privacy screen on the display or an application that dims areas not in your gaze. Microsoft has patented a system that it believes is superior to both because it makes the document fully encrypted and illegible at all times—apart from the section you're directly looking at on the display.
The patent's details (via Windows Report) are like all such publications, in that specific details on exactly how everything works aren't covered. Instead, a broad overview of the nature of the technology is given and what Microsoft has proposed is a system that takes documents you're working on and encrypts them in such a way that the contents are secure but the overall structure of the text remains the same.
