Figure 5.2.2.5[White to move]

Here you want to see two things: that White is stopped from playing QxQ only by Black’s bishop; and that White is stopped from mating on the back rank with his queen or rook only by Black’s bishop, which can interpose at f8. Ordinarily you might therefore like to play QxQ, but not here; for once Black replies BxQ, his king has a flight square and Rd8+ just results in Kg7. So again try turning it around and playing the mate threat first, expecting it to fail—but to leave the queen loose. Thus White starts with Rd8+; after Black plays Bf8, his only legal move, White has QxQ.

This position arose when Black played Qxb2. The move seemed safe because the queen was guarded by its bishop from g7. A valuable lesson recurs: if you send a piece on an adventure and rely on a fellow piece to protect it on its new square, pause to ask what else that other piece already is protecting—any other pieces, and also any squares on which the enemy would be able to mate.