Pin
A pinned piece cannot move safely because moving it would expose a more valuable piece or the king behind it.
What to look for
Look for pieces lined up with the king, queen, or rook behind them. Then attack the pinned piece or add pressure.
Recognize the mechanism
Trace attacker, pinned piece and rear target along one line. Identify whether moving the front piece exposes the king or only a valuable piece, then calculate how you can increase pressure without releasing that alignment.
Common practical mistake
A relative pin does not make every move illegal. The defender may accept a material trade or answer with a stronger threat.
Training goal
Learn to exploit pieces that are stuck defending something behind them.