Figure 2.3.6.4[Black to move]

This time Black has no bishop checks (except the useless Bxf2+), so just ask what else his bishops can attack. You see then that BxN threatens White’s queen. The threat amounts to nothing yet because the bishop wouldn’t be protected against QxB and because it has nothing else to attack at the same time. But the king is not far away, and if it could be moved onto h1 Black would have a classic triangular fork with BxN. Consider any checks Black can give and how they might change the board. There's Bb6xf2, which loses the piece, and then the more interesting Qxg3+. Can White reply f2xQ? No, because the f2 pawn is pinned. White therefore would have to play Kh1. Now BxN attacks both king and queen—and thanks to the previous move by the queen, the bishop now would have protection at f3. So White responds to BxN with QxB, and Black replies with QxQ+.

And what then? White has to move his king to h2. (Moving it to g1 is also a possibility, but eventually leads to mate for Black.) Black keeps the offensive pressure on with Bxf2. White has no appetizing options in reply. Black threatens to mate with Qg3+ followed by Qxh3#; White can prevent this by using his rook to take the bishop on f2 or by playing Rg1, but either way he loses the exchange. Seeing the initial forking idea here was the key point, but playing through the rest of the sequence in your mind’s eye will be a useful exercise in extending your range of vision.

In summary, then, here is the most likely sequence: 1 …Qxg3+; 2. Kh1, Bxf3+; 3. QxB, QxQ+; 4. Kh2, Bxf2; 5. Rg1, Bxg1+; 6. RxB.