Figure[Black to move]

Black’s dark-squared bishop is poised to move to c5, pinning White’s queen. Any problem with this? Yes: White replies NxB. Black needs to get rid of the White knight on b3 before the pin will work. He can't exchange away the White knight because no Black piece attacks it. So now instead of a capture consider a threat. Black plays a5-a4, and White’s knight must flee unless it is to be sacrificed with only Black’s pawn as compensation. Accepting that sacrifice actually would be the better part of valor here, since if White does move his knight—say, to d2—Black follows up with Bc5, pinning and then taking White's queen with support from the rook on c8.

The lesson is to think carefully about the pieces that guard the square you need. Exchanging them away or threatening them with pieces less valuable than themselves may leave the pinning square (or forking square, or whatever square) weak enough for occupation.

By the way, White has another line of reply that produces a similar material result. After Black starts with a5-a4, White can try a counterattack: a2-a3, making a hostage out of Black's b4 bishop. The bishop retreats to e7; this buys time for White to take his queen out of danger, too, and with a threat: 3. Qa7. Again Black retreats: 3. ...Bd8. Remember that Black still has a pawn on a4 threatening to take the b3 knight; so now White plays 4. Nc3xa4, removing that pawn. Alas, Black has another pawn at b5, and uses it to recapture: 4. ...b5xNa4. White can take that pawn with his queen, but then Black also has Nc4xe5. Black ends up with a knight and a pawn in return for two pawns. (White has to play 4. Ncxa4, by the way, in part to protect against Black's use of b6 to launch a bishop fork against White's king and queen—but this is getting rather involved.)

That last paragraph assumes that White deals with the Black pawn on a4 by capturing it. You might wonder whether White could instead just move the b3 knight to a safer square, such as c1. But if play goes as we've been describing, that knight move has a troubling consequence. Black can then move his bishop (now on d8, remember) to b6, which forks the White king and queen. By instead playing Nxa4, White throws an attacker at the b6 square and so heads off the fork. Black still takes the knight with his b-pawn, but then White has a move to take his queen out of forking range.