Figure 5.2.1.15[White to move]

Now consider our theme from a defensive standpoint. White looks for anything he can take and sees that he has the possible capture Bxa5. The pawn is guarded by Black’s rook, but White attacks the pawn twice—with his bishop and with the queen behind it. Yet there is a fly in the ointment. White is imagining that Black will hesitate to play RxB because then White has QxR. But when you have that train of thought—in other words, when you imagine that you need not worry, because piece A will protect piece B—pause to ask whether piece A already is protecting anything else. In this case the queen presently is the sole defender of White’s knight at d4. Black therefore can reply to Bxa5 with BxN. If White recaptures on d4 with his queen, he loses his bishop at a5. Thus White foresees that an apparently safe capture would leave him with an overworked piece, and so isn't safe after all.

You might have supposed that play would go 1. Bxa5, RxB; 2. QxR, BxN—and concluded that White ends up with a pawn and rook for two minor pieces. As we see, though, Black can do better by playing BxN a move earlier than that. Remember this idea: captures may be forcing moves, but they don’t necessarily call for a recapture on the spot; a fresh capture or threat elsewhere also is a possibility. Your opponent has choices of his own to make about move order. In this case White's initial capture 1. Bxa5 suddenly leaves his queen overworked. Black exploits it best by starting with the less valuable of his attackers—his bishop on f6 rather than his rook on a7—and inviting White to capture it.

A larger lesson of the study: be cautious about overextending your pieces. If you're thinking about sending a piece into enemy territory (the temptation to snatch a pawn there is common), consider carefully how you will protect it; if your plan is to guard it with another piece, ask how secure the protection will be. One danger is that your guard can be taken in an exchange, leaving the advanced piece loose. Another danger, seen here, is that your guard suddenly will become unable to do defensive work elsewhere, in effect leaving some other piece or pawn loose. One way to stay alert to these dangers is to be aware of any threats your opponent already has against any of your pieces.