Figure[Black to move]

Once again the first order of business is to create a pin. Black scans the lines leading from White’s king and finds no useful targets, so he considers checks he can inflict and their consequences. He has one with his rook and a few with his queen. The most natural queen check to consider is Qf3+, both because it is safe and because it combines the attack on the king with an attack on the loose bishop at e3. The move forces White to reply Bf2, as this blocks the check and also allows the king to protect the bishop; if White instead moves his king, he forfeits the bishop. This is the difference between Qf3+ and, say, Qf5+. It’s a study in the usefulness of seeing loose enemy pieces.

So the bishop interposes and immediately is pinned, and the question now is whether it can be taken down. Both sides will rush pieces to the scene of the pin; the question is which way the balance of power will tip once each side’s resources are exhausted. Black’s first move is Rf8. Moving the rook behind the queen effectively means that White’s bishop is attacked twice. (It is important to realize that one way to increase pressure against a target often is by adding another piece behind the one inflicting the pin.) Indeed, since the king is the bishop’s only guardian, Black’s threat is even greater: QxB would now be mate.

Next question: how can White beef up the bishop’s protection? He has two pieces left: his queen and rook. The queen moves to b6 and again the forces are balanced; the bishop is attacked twice and protected twice. So Black throws yet another piece at it with Ng4, where the knight takes the square that Black’s queen originally occupied. White is out of answers. He can’t get his rook into position in time to be helpful. The bishop is attacked more often than it is guarded, and must be lost. Black will take it with his knight on the next move.

Okay; now backtrack a minute. When Black played Rf8 on his second move, he also had the option of adding to the pressure by playing his rook from d8 to d2. But this would be less favorable. The short reason why is that Rd2 leaves Black's back rank bare, which thus allows White to reply with check: Rb8+. Black can block the check by interposing his knight on g8; White takes it with his rook, and Black recaptures; and now White has another check with Qc8 (at this point White has to be careful not to give Black a move he can use to play Rd1#; he has left his own back rank bare). Things can get messy here if the Black king tries to run, but Black's more likely play is Qf8, blocking the check. After White forces an exchange of queens, Black ends up ahead the exchange—not as good a result as we saw in the main line. Even without working out all these details, though, you should be able to see that on principle it's probably better to let the d8 rook do its work while staying on the back rank (i.e., by going to f8 rather than d2) and not leaving the king open to attack.

So: 1…Qf3+; 2.Bf2, Rf8; 3.Qb6, Ng4; followed by NxB.