White’s rook aims down the d-file, where Black has a pawn and then a queen behind it. The pawn almost is pinned, leaving the knight on e5 loose; the only obstacle to the pin is that White’s own knight is in the way. Toy with vacating it violently so your opponent will have to spend a move on his response. The simplest way to move a piece violently is with a capture, so White looks for things his knight can take and plays Nxb6. Indeed, this not only is simplest but also is most profitable: if Black decides now to move his queen and avoid the pin, White at least has used the threat of the pin to win himself something—a pawn. If Black instead opts to recapture with c7xN, the pawn on d6 is pinned and White can play QxN+. If Black then recaptures with d6xQ, White plays RxQ, winning back his queen and again netting a pawn with the sequence. Do not turn up your nose at the gain of a pawn. But the larger lesson to take away from the position is the look of the d-file, the pinning possibility it contains, and the implications of the pin for the knight on e5. It would be easy to miss this in game conditions.