A Discovered Attack occurs when you move one piece out of the way, revealing an attack from a long-range piece (Rook, Bishop, or Queen) that was standing behind it. It is essentially a two-for-one move.
This tactic is devastating because the piece that moves can create its own threat, while the revealed piece creates another. The opponent often cannot defend against both simultaneously.
If the revealed attack is a check, it is called a Discovered Check. This is even more powerful because the opponent is forced to respond to the check, allowing the piece that moved to capture almost anything on the board for free.
The most extreme version is the Double Check, where both the piece that moves and the piece behind it attack the King. In a double check, the King MUST move, as capturing or blocking one attacker still leaves the other active.
Strategic success with discovered attacks depends on alignment. You should intentionally place your long-range pieces on the same line as the enemy King or Queen, even if one of your own pieces is currently blocking the path. This creates a hidden tension that can be unleashed at the right moment.
Defensively, you must be extremely cautious when an enemy Rook or Bishop is aimed at your high-value pieces. Even if the path is currently blocked by your opponent's own units, any move they make could suddenly turn that alignment into a game-ending discovery.