Trébuchet

The trébuchet is an extreme form of reciprocal zugzwang. Zugzwang means that the obligation to move makes a position worse. In the trébuchet, that obligation decides the game directly: the player to move loses with correct play.

The typical formation contains one king and one pawn for each side. Each king attacks the opposing pawn while also protecting its own. Neither king can capture immediately because it would move onto a square controlled by the other king. The player to move must nevertheless move the king, stop protecting the pawn, and allow the opponent to capture it.

After winning the pawn, the other side keeps its own pawn and can normally guide it to the last rank, where it becomes another piece. The mechanism is not a hidden tactic. It follows from the geometry of the kings and the obligation to move.

Not every mutual zugzwang is a trébuchet. The name refers to this characteristic pawn-ending pattern in which both kings attack the opposing pawn and the move determines which side must yield first.