Black has just interposed his queen between his king and White’s queen. White momentarily has Black’s queen pinned, but he will have to act fast to make anything of it; Black is ready to play QxQ, and even if White guards his queen (say, with Rc1) the resulting exchange will wipe out the pin and any potential it held. When your queen pins your opponent’s this way, a first thought should be to aim another piece at (and through) the pinned queen, so that QxQ becomes more costly for your opponent to play. The natural method here would be to move the rook over with Rb1. This might not look like much, since it just positions the rook to attack the pawn on b7 if Black moves his queen; but of course the real significance of the threat is that it targets the b7 square, which is adjacent to Black’s king and already attacked by White’s knight. So if Black plays QxQ he suffers not just Rxb7+ but Rxb7# (mate). It's our now-familiar concept: Rb1 cross-pins Black’s queen to a mating square.

Black can avoid immediate doom by replying to Rb1 with Nd7, which protects his queen and gives his king a flight square. But then White plays Qf5, preparing to take Black’s knight on its new square. It's a little surprising to see White move his queen away, but this turns out to be surprisingly effective. First, notice that White really doesn't need his queen on c5 to pin the Black queen anymore. White's rook has the situation under control: if Black moves his queen, Rxb7+ follows for White, with the knight on a5 giving cover; and then Black either loses the queen or gets mated. But if Black doesn't move the queen and lets White play QxN he also has to worry about White's queen then adding to the mating attack against b7. And meanwhile from f5 the queen protects the b1 rook against QxR+ by Black. All Black can do in reply to Qf5 is guard his knight with Rd8 and permit White to play RxQ. It avoids the immediate threat of mate, but after Black recaptures, White has won a queen for a rook.