Figure[Black to move]

You should have no trouble seeing that Black’s queen and knight both attack g2, that Black nearly is ready to mate there with Qxg2, and that White prevents this with his queen on d2. The question is how you best can take advantage of the constraints this pattern imposes on White’s queen. The only thing the queen protects that you can take is the pawn on d4. That would not be a bad gain by itself, but in any event it is not just any pawn; once Black takes it with Rxd4, his rook now adds to Black’s offensive thrust by bearing down directly on White’s queen and the rook behind it. So consider White’s possible replies to Rxd4 and their consequences.

(a) If he plays QxR, Black mates with Qxg2.

(b) White might search for a square where the queen is safe and still protects both g2 and the rook on d1, and so play Qc2. But then Black plays RxRd1+ and mates next move no matter what White does.

(c) White’s best move is to evacuate his queen from d2 and try to take out the mate threat on g2 directly by playing QxN. Black doesn’t play QxQ now because White’s queen still is guarded by his bishop. He plays RxR+, winning the exchange and gaining a tempo he can use to keep his own queen safe after White’s king is forced to h2.

This position could have gone in the next chapter, which covers threats against a guard; Black’s Rxd4 was an attack on White's queen, which was protecting a mating square. But the position also fits here because of the (b) line described above: Rxd4 adds a second threat of mate—RxR—on top of the existing threat of Qxg2. White’s queen is responsible for preventing both. If White isn’t careful, one of the threats will succeed; and even if he is careful he loses material. We see again a standard method for overworking a piece: observe that it is preventing mate; then make threats on squares that it appears (perhaps deceptively) to be able to guard.