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Underpromotion
What it is
Underpromotion is promoting a pawn to a knight, rook or bishop rather than the usual queen, because in that specific position the 'weaker' piece is actually stronger — delivering a fork or mate, or avoiding a stalemate.
When to use it
Use it when a new queen would allow stalemate or fail to a check, or when a knight promotion comes with check or a fork that a queen cannot deliver.
How to use it
Before promoting automatically, check what the new piece does. If a queen stalemates or is met by a defense, ask whether a knight's checking geometry or a rook's mate net wins instead. Promote to the piece that finishes the game.
Worked examples
Watch the coach play the tactic from real master games.