Explanation

When an attack is defended with the help of another piece by blocking or protecting, that helping piece is drawn into the battle. If you block an attack, the blocking piece is usually pinned, so it is restricted in its movement. Similarly, a protecting piece is restricted in its movement because it is bound to its defensive task. You can think of the battlefield extending, as it now includes the additional helping piece. This opens the chance for a second wave of attack.

A pinned piece can't move without losing material. It can't run away, so defending against another attack on the pinned piece might not be possible.

In the first diagram, the queen pins the rook against the king. This is an absolute pin, and the rook is not allowed to move. White can attack and win the pinned rook with the pawn move indicated by the arrow.

In the second diagram, the bishop pins the knight against the queen. This is a relative pin, so the knight is allowed to move out of the pin. However, doing so would expose the queen to the attack of the bishop. White can attack and win the pinned knight with the pawn move indicated by the arrow.

Examples

Black can win by attacking the pinned knight with 1...Kd4, for example 2.Kf4 Rxe3.

Black is a bishop down in this position, but can regain the material with an attack on a pinned piece.
White's bishop g5 is pinned by the black queen against White's unprotected queen.
Black can attack the pinned piece with 1...h6, and White has no way to defend the bishop.
Note that 1...f6 would not have been the correct move, as White would then be able to defend with an intermediate check 2.Qe6+ (or 2.Qc4+).

White can win with 1.Bd5!. This is not a sacrifice because the bishop is pinned, for example 1...Bxd5 2.Rxe8+.

Related motifs