A queen or rook on a rank where it is masked by a pawn is a good setup for a discovery; if the pawn moves (whether forward or diagonally), the attack is unmasked. Here Black's queen is on a5, masked by his pawn on d5. The pawn can’t advance on the d-file, but it can make a capture on c4, thus clearing the queen’s path across the fifth rank and also launching an attack of its own on White’s d3 bishop. All that needs to be considered is the queen’s target. White has a bishop on g5. It’s not loose, so if the queen were unmasked it wouldn’t be able to make the capture. But the bishop’s only guard is the knight on f3, and a piece that is protected only by another piece often can be weakened or left loose by a preliminary exchange—a capture of the guard, or a capture of the target that requires its guard to recapture and then be left loose. Here Black thus starts by taking the bishop with NxB. NxN follows for White—but now he has a loose piece on g5. Black plays d5xc4, winning material no matter what White does.