Hint:(spoiler break)
(spoiler break...)
1) Figure out what you can mark.
2) Figure out what keys could mark that.
why does pressing laterally any key on the keyboard let you unmark a tile?
also ubj qb lbh trg erfbyhgvba bs terngre guna bar gragu?
it seems like you would have to figure that out before even the hint is revealed.
nevermind, I had simply figured out the last rule before the the forth one.
