What does White threaten? His rook attacks the bishop on e6. His queen and bishop attack the knight on d5—which in turn is protected by Black’s queen and bishop. The balance of pressures on the knight are even, but they won’t be if one of the knight’s guards is exchanged away. White captures one of the knight’s guards with RxB, once more sacrificing the exchange temporarily. No response Black can make will save the knight; no matter what he does, it will be two attackers against one defender. But Black’s response does have significance, for if he plays QxR—and here he has nothing better—he finds his queen fatally pinned a move later by White’s BxN. (The potential for this should have been clear from the alignment of two of Black’s pieces with his king.) Black can swing his rook over to e8 to protect the queen, but he still ends up trading his queen, a knight, and a bishop for White’s bishop and rook.
Is it surprising to hear that after White plays 1. RxB Black has nothing better to do than play QxR and let his queen be pinned? Look at his predicament: his queen is attacked by White’s rook; if he doesn’t take the rook, then after White plays BxN—winning his second piece—he will have the kernel of a discovered check and both pieces will be well-guarded. Sooner or later Black has to sacrifice his queen to forestall checkmate, which now will come soon enough anyway.
The position is an example of a theme we will visit several times in this section: the importance when removing the guard of remembering to consider how other tactical tools might be put to work as well. Exchanges to remove a guard (or for any other purpose) often open lines that create openings for pins and forks. In fact this position could as easily have appeared in the chapter on creating pins. Seen from that standpoint, this would be a case where tracing the lines out from Black’s king turns up, via BxN, a potential pin of a bishop—an unsuitable target. So White swaps it out for a queen with RxB, QxR. Now the pin succeeds, as after BxN the bishop has protection from its own queen.