Explanation

As with pins, only bishops, rooks, and queens can skewer, because these pieces have a long-range attack. The difference between a pin and a skewer is mostly in how the opponent can react to the attack. With a pin, the opponent doesn't want to (or is not able to) move the attacked piece, whereas with a skewer the opponent prefers to or must move it.

In the first diagram, the bishop pins the rook to the king. This is an absolute pin, so the rook can't run away and can be captured on the next move.

In the second diagram, the bishop skewers the king and rook. This is a check, so the king must move, exposing the rook.

Sometimes a skewer can win material immediately. All puzzles in this level are cases where you can win material with a skewer.

Examples

White can win material with the skewer Rf8+.
Black cannot block the check, and has to move with the king out of the check. This reveals the rook on a8 behind the king, which White can capture with Rxa8.

Black can regain material with the skewer Be1.
Black attacks the rook on d2, and the unproctected knight on c3 is behind it.
White cannot move the rook to a square where it protects the knight, as both c2 and d3 are guarded by the queen on f5.

How to spot a skewer

Whenever you can make an attack with a long-range piece, quickly find any skewers by also considering what is behind the attacked piece.

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