We turn now to a few mechanical issues that can arise in building relative pins. Most of these involve familiar ideas; only the settings are new.
The point to notice here is that Black has his bishop and rook arrayed on the same diagonal. White can reach the same line with the c1 bishop. For the idea to work, Black’s bishop needs to be replaced with something White’s bishop can attack without fear, so White performs an exchange: 1. RxB, NxR. Now Bg5 pins the knight that has moved to f6; if the knight moves again, Black’s loose rook on d8 is lost. Black has no way to add protection to the knight (if he plays his rook to d6 it gets taken), so White gains the piece a move later.
You also could have seen this idea by considering any captures you can make. White only has one—RxB—so analysis is simple. The important thing to see is not just that Black replies NxR but that this puts a knight and rook on a dark-squared diagonal, poised to be pinned by White’s dark-squared bishop.