Start by looking for a piece in a pinnable position. Here it’s Black’s bishop on e7. When a bishop is on a diagonal with its king it probably cannot itself be pinned, but often an exchange can be used to replace it with a better target. Here White considers each piece he can use to take the bishop and picks the least valuable one—his knight—to do the dirty work: 1. NxB, RxN. (Again, Black can’t recapture with his king because e7 still is under attack.) Now Black’s rook is ready to be pinned by Bg5 and taken a move later: White has two pieces trained on the rook, so again the king—the only piece protecting it—will not be able to recapture after RxR.