Skewer
A line attack that forces a more valuable piece to move, exposing a less valuable piece behind it to capture.
A skewer is sometimes called a reverse pin. The attacker threatens a more valuable piece along a line; when that piece moves away, a less valuable piece behind it is captured. Like the pin, the skewer is the work of long-range pieces: bishop, rook, or queen.
The most common skewer is on the king. A check on the king along a file, rank, or diagonal forces the king to step aside, revealing the piece behind it. If that piece is the queen, the result is immediate material gain. Many endgame combinations turn on a single skewer of the enemy king and queen along a long diagonal.
Skewers and pins are mirror images. In a pin, the back piece is more valuable and the front piece is paralysed; in a skewer, the front piece is more valuable and the back piece is the prize. The same long-range piece can deliver either motif depending on the configuration.
Practical chess offers many skewer opportunities in queen-and-king endgames, where the queen and an enemy piece often end up on the same file or diagonal. A correctly timed check that skewers them wins the game. The defence against a skewer is the same as against a pin — anticipate the line and block, or move the back piece before the front one is forced.