We still are looking for targets and vulnerabilities in their protection, only now the chain to be examined is a little longer. What does White threaten? His bishop attacks the pawn on c4, and his rook attacks the bishop on d4. Black’s bishop is the prize. It is protected by a knight. So White hacks away at the chain with Bxc4+. This attacks Black’s knight, which is not worrisome to Black in itself since the knight remains protected by a rook; the threat is that if the knight gets taken, the bishop on d4 will be exposed. Normally Black could deal with this by just moving his bishop, but not here—for the beauty of Bxc4 is that it checks Black’s king and leaves him no time to save the bishop. He spends a move relocating his king, and now White plays BxN. In effect White has simply played a bishop fork to take out the Black bishop's guard.
At this point Black could recapture with RxB, to which White would reply by likewise playing RxB; Black then finishes with Rxb2, winning back a pawn but losing a piece with the sequence. Black can do a little better, though. After 1. Bxc4+, Kf8; 2. BxN, each side has a rook that attacks the other side’s bishop. But instead of trading pieces Black can play Bxb2; this saves his bishop and picks up a pawn. Now White has to play Rd3 to protect his own bishop, and this gives Black time to finish with Bxa3, picking up still another pawn. Black thus ends up with two pawns to help offset his loss of a knight, though White has a won game in any event.
The process of removing the guard often involves exchanges that are a wash in themselves but that leave other pieces loose for the taking. If the guard is protected by a second guard and so forth, you sometimes can start by attacking the last link in the chain, exchanging your way toward a target that will be left loose in the end. But those preliminary exchanges must be studied carefully. They are partially forcing moves, but only partially; your opponent is likely to reply to your captures with predictable recaptures, but in principle it usually is open to him to decline the recapture and play some other threat that takes the initiative or otherwise ends your threat. That is why it is so valuable to include checks in your sequences when possible, as White does here with Bxc4+. The check forces your opponent to pick from a short menu of replies, and keeps you in control of the action.