Pin to win
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If the more valuable piece behind the attacked piece is the king, moving the attacked piece out of the piece would be illegal, because putting yourself into check is not allowed. This is sometimes called an "absolute" pin, while other pins are sometimes called "relative" pins.
Only bishops, rooks, and queens can make a pin because these pieces have a long-range attack.
A pin can often be useful to restrict the mobility of the opponent's pieces.
Sometimes a pin can win material immediately. All puzzles in this level are cases where you can win material with a pin.
Black can pin the white queen with Rd1.
Now White's queen cannot move "out of the pin", because it would leave White's king in check.
The only legal moves with the queen are Qf1 and Qxd1, in both cases Black can win the queen for the rook.
White can pin the black rook against Black’s queen with Be5.
If Black move's the rook out of the pin, the queen on b8 will be exposed to the bishop's attack.
Whenever you make an attack with a long-ranged piece, also consider what is behind the attacked piece, to quickly find any pins and skewers.
An attack on a piece is called a skewer, when moving it out of the attack would expose a piece (of equal or less value) behind it.
The difference between a pin and a skewer is mostly in how the opponent can react to the attack. With a pin, the opponent does not want to, or is not able to move the attacked piece, whereas with a skewer the opponent prefers to move it.
Sometimes a pin does not win material immediately, but because the pin restricts the movement of the pinned piece, additional tactics are possible.
Attacking a pinned piece can win material.
If the pinned piece is protecting another piece, that apparent protection can turn out to be false, and the pin can be used to win material.