A pawn in the middle of the board with opposed rooks on either side of it, like White’s pawn here on d4, is a natural candidate for a pin; what makes the pin work for Black, of course, is that White’s rook on d1 is loose. This amounts to an offensive opportunity: the d4 pawn is defensively inert, so Black should search for ways to seize whatever it is meant to protect. It is meant to protect White’s knight on e5, so study the knight and ask whether it can be taken. Not quite: if Black plays f7-f6, the knight simply moves. And Black can’t take the knight with his h8 bishop because the knight has one other guard in the pawn on f4. What is to be done? Since Black has the prospect of winning a whole piece, he can afford to make a sacrifice to get rid of the f4 pawn. He simply takes it with Nxf4. After White recaptures RxN, White’s knight has no guard other than the pinned d4 pawn, which is to say it is loose. Black plays BxN and wins a pawn with the sequence.

The structure of the position is simple: a piece that White seems to protect with two pawns actually is protected by just one. So if Black takes the pawn, allows a recapture, and then takes the piece the pawn was supposed to protect, he ends up ahead a pawn.