Discovered Attacks and the Double Check
The ambush behind your own piece
A discovered attack happens when you move one piece out of the way to unleash an attack from a different piece standing behind it. The magic is that two things happen at once: the piece you moved can make its own threat, while the piece it uncovered makes another. Your opponent suddenly faces two problems created by a single move, and as with a fork, they can usually only solve one.
The reason this is so deadly is that the moving piece is free to do whatever it likes. It can capture, it can threaten, it can run to safety, all while the hidden piece behind it does the real damage. A discovered attack that uncovers a threat on the queen while the moving piece grabs a rook can win enormous material in one stroke.
The discovered check
When the uncovered attack is a check, the tactic becomes vicious. The opponent must answer the check, so they have no time to deal with whatever your moving piece just did. That is how a discovered check can casually capture the queen: the king is in check from the piece behind, the opponent is forced to respond to the check, and your roving piece walks away with the prize.
Double check: the most violent move in chess
There is one move no defense can blunt: the double check, where moving a piece gives check from that piece and from the one behind it at the same time. Two pieces are checking the king at once. You cannot capture both, you cannot block both, so the only legal reply is to move the king. The king must run, no matter what else is hanging on the board, which is why double check often leads straight to checkmate. When you see the chance for a double check, slow down and look hard, you may have a forced mate hiding in plain sight.