Examine Black’s king and be struck by its limited range of motion (none, thanks to the attack on g8 by White’s bishop). In these circumstances any check becomes a mating threat; White’s RxN would be mate except that Black’s rook on c8 guards the mating square. Can White somehow be rid of the Black rook? He can’t capture it, and if he attacks it Black might just leave the rook where it is, comfortable in the knowledge that it is protected by its queen—and that if it is replaced by its queen, the back rank still is defended. But the calculus changes if White can attack Black’s rook while also creating another threat. So then notice another significance in the fact that White’s bishop on b3 is aimed at g8, next to Black’s king. It gives White a classic queen fork of a Black piece and mating square with Qc4. Ordinarily the move wouldn’t look like much because Black can just play RxQ. But here that capture is exactly what White wants Black to play (and what Black does not want to play) because it results in a mate for White with RxN. It does not matter what Black does, of course, since if he plays anything other than RxQ (e.g., Qe6) he soon ends up mated by Qg8#.