Black’s rook on a8 is under attack, as is his queen; but consider your offensive options before worrying about it. Take apart the knot of pieces near White’s king by asking methodically what attacks what (and what defends what). There are no fewer than four pieces that Black's queen might take. The most attractive capture, of course, would be QxQ. What prevents it? White’s bishop on g2. So now you ask what else the g2 bishop protects and are led to the knight on h3, which you can take with RxN+. The beauty of the move is that White has no choices to make in reply. His king is under attack and has no flight squares, nor does Black have anything to interpose, so BxR is compulsory. Now White has QxQ+ in return for his sacrifice of the exchange, and an eventual forced mate.

Notice from the beginning the telltale visual pattern in which the White bishop supplies protection two fellow pieces in a Y formation—a sign of weakness.

And again we see the importance of move order. It might have seemed plausible, but would have been mistaken, for Black to start with QxQ, inviting BxQ and then planning to win a piece with RxNh3+ followed by RxBf3. Actually, though, this wins less than a whole piece. In reply to 2. RxN+ White has 2. ...Kg2 and suddenly both of Black’s rooks are under attack—one by White's king and the other by White's bishop; and now the king guards the bishop, making it safe. Black plays Rh3xBf3, but then White replies KxRf3 and Black only ends up winning two pieces for a rook. He still is losing, since he was laboring under a larger material deficit than that from the outset.