Here is another way it can go. Black’s rook masks his bishop, which otherwise would have an open line to White’s king. What should Black do with his rook? The queens are off the board, so Black looks for pieces he might attack. There are two: White’s knight and rook. Which is the better target? The knight, because it is unprotected; if Black plays Rb2+ and then RxR, he just ends up exchanging rooks after NxR. The winning move thus is Ra2+ (or Rf3+), followed by RxN after White saves his king.