A pawn one step from promotion warps every calculation on the board, because trades stop being equal: any piece that dies stopping a promotion effectively costs its full value, and any defender that gets deflected from the promotion square loses the game on the spot.
The standard trick when a defender guards your queening square: offer a trade ON an adjacent square, so that capturing your piece recaptures into a brand-new queen. The defender faces two bad options: take and hand you the queen, or retreat and give up the square anyway.
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White's pawn is one step from queening, but Black's rook guards c8 along the back rank.
The rook guards c8. Force the issue.
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