Figure[Black to move]

The cluster of pieces around White’s king seems full of potential for Black. But the most important piece is elsewhere; it is the bishop on g6, because it pins the pawn on c2. If you are thorough in following out the lines from the king you will not miss a long-distance pin like this. Why is it so critical? Because now when you study the cluster around the king you realize the c2 pawn is doing no defensive work; anything it appears to protect, it doesn’t. What it appears to protect is the pawn on b3, which is attacked twice (by Black's a4 pawn and his rook on c3) and in reality is defended just once (by White’s bishop). Black therefore is free to play a4xb3; and if White replies Bxb3, Black wins the bishop with RxB+. White cannot recapture and will suffer more losses soon. (Slightly better for White after Black’s initial capture would have been Ka1, but it’s still a lost position; Black then plays Bb4, unmasking a rook pin of White’s bishop on a2 and preparing a discovered attack against the White rook on d2 and—behind it—the other rook on e1: e.g., Rc3-c8.)