Discovered check (2 moves)
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A discovered check can be especially strong if the discovering move is a forcing move, for example a capture, or an attack. In that case, the opponent often cannot recapture, or defend the attack piece, and defend against the discovered check at the same time.
In this sense you can compare a discovered check with a fork where you attack a piece and give a check. For such a fork, one piece attacks and checks. For a discovered check, one piece attacks, and another piece gives check.
A special case of a discovered check is the double check, where you give check with one piece, and discover a second check from another piece.
Black can win with the discovered check Bc3+.
The rook on e8 gives check, and the bishop on c3 attacks the rook on b4.
White is unable to defend against both threats with one move. Note that Re4 doesn't work here, as the rook is unprotected on e4.
Because your opponent has to deal with the check first, the discovering piece gets a free move, and can move to an otherwise inaccessible protected square, as in the following example:
White can win with the discovered check Na7+.
The queen on b5 gives check, and the knight on a7 attacks Black's queen.
Black cannot defend against both threats at the same time. Rxa7 is an illegal move as it would leave the king in check. Qc6 doesn't help as the queen is insufficiently protected on c6.
Whenever a possible check is blocked by one of your own pieces, think about ways to move that piece and make a discovered check. Your opponent will have to deal with the discovered check first. You get a "free" move with the discovering piece, and can consider possible sacrifices. With some practice, you will be able to spot blocked checks quickly, and grab your chance if a discovered check is possible.
A discovered attack is a move by one piece that reveals an attack by another piece.