Shaded arears on the grid?

[pcuser]

Hi all,

I've noticed for some time now that if you mishit a square on a line, you create shaded areas in the puzzles. Sometimes it's just a square and other times it can take up most of the grid and the clues. I've never stopped to try to figure it out as I'm trying to finish the puzzles. I have noticed if you then hit outside the puzzle, these shaded areas vanish. Anyone know what this feature is? Any info would be appreciated.

Thanks,
pcuser