Black is down a piece; how should he proceed? He might, of course, consider any checks he can inflict and whether they lead to tactical opportunities. He has a queen and a rook to examine. Qh3+, Qxh2+, Qf3+, and Qe2+ can be dismissed quickly, as they all lose the queen with nothing to show for it. Then there is Rc2+, which at least is a safe move. Consider what White would do. Playing Kh1 or Kg1 won't be good for him; Black then looks for his next check and finds that he can end the game with Qxh2#. And White can’t move the king to h3 or f3, nor can he capture Black’s rook. So he would have to interpose something. Be2 wouldn’t make sense, since Black just plays RxB+. White instead would need to play Rd2, using his rook to block the check while protecting it with his queen. The key point to spot is that this interposition, like any other, leaves the guardian pinned.
The next step: attack the pinned piece again. Consider every way you can do it; find Qd1. Now White is in terrible trouble, for his rook is in a cross-pin. If he plays RxR, his queen is lost to QxQ. Anything else he plays is no better: if he moves the king, RxR wins the rook and then the bishop (to QxB) a move later; if White plays Be2, RxR again wins the rook for Black and then the bishop a move later, since it would be pinned. This position is worth some study, as the key moves and their implications are a bit counterintuitive.