Where does White have a target? In the Black knight on f4. White attacks the knight with two pieces, and it also is protected twice: by the pawn on e5 and by the bishop behind it on d6. Those guardians are themselves guarded, the e5 pawn by the queen as well as the bishop, and the bishop by the queen as well as the pawn on c7. The first point to grasp is that so long as Black’s knight is guarded by a pawn, White is going to have a hard time turning much profit by capturing it; whatever he uses to take the knight will get recaptured by the pawn, which then can be taken without great loss to Black. So the first order of business is to force Black to substitute a piece for the pawn. White plays d4xe5, and Black has to recapture with his queen. (Do you see why? If he recaptures with his bishop, the knight on f4 now has just one guard and is taken next move.) Now Black’s knight is protected by two Black pieces. Both guards are protected, but that isn’t the point. The point is that White now can perform a classic exchange of minor pieces with NxB, forcing Black to retake with the queen or with the pawn on c7. Either way the protection of the knight on f4 has been whittled down to just the queen. White plays BxN, and now Black dares not recapture because he would be sacrificing a queen to the cause.

We already knew that pieces defended by other pieces are potentially vulnerable because you can take their guardians without much (or perhaps any) sacrifice; pieces protected by pawns are hardier because any sacrifice you would make to get rid of the pawn tends to be such a poor trade. This position shows one way to take advantage of the point. If the pawn that protects a target can be taken by one of your pawns, resulting in a recapture that causes the target to instead be defended by an enemy piece, you may then be able to win the target by capturing its new protector without sacrifice.