Chess PatternsFork
One piece attacks two targets at once, and the opponent can only save one of them. Knights are the classic forking piece, but every piece can do it.
How it works (spoiler)
The knight checks the king and hits the rook in the same move. The king has to step aside, and the rook in the corner falls. A check is the strongest prong of a fork: the opponent has no time to save the other piece.