White’s rooks are arranged on the same diagonal and thus could be forked with Bxb2. But that won’t do because White is on the verge of mate with Qa8, Qxc7, or Ra8. Black’s next move therefore needs to be a check that forces White's reply. He has four. Two can be dismissed quickly: Rd1 just loses the rook and Qxg2 just loses the queen. A third check, Bxh2, is a little more interesting, as White wouldn't particularly want to reply KxB; for Black would then have the queen fork Qh6+, compelling White to trade queens to get out of it. Instead White would move his king to f1. Now Black has the queen fork Qxc1, again forking White's king and queen—but this time without protection. So White plays QxQ. Black has taken a pawn and a rook but has given up his queen for the sake of disrupting the mate threat. So all this is possible, and might even seem an appealing reprieve from execution—but hold! For there is yet one more initial check for Black to consider.
Starting with this last remaining check, QxR+, turns out to be most interesting of all. White replies QxQ, letting go of his mate threat. Now what would be possible? White’s queen and rook would be on the same diagonal, so Black toys again with Bxb2. Of course White could just play QxB. But again you consider what lines would have been opened by all this and what checks Black would then be able to inflict. As for open lines, Black’s bishop would have moved off of the e-file, creating a new path for the rook on e8; now the rook could move to e1—which would be mate! So in reply to the forking move White is obliged to move his queen and let his rook on a3 be taken by Black’s bishop. Black emerges with two rooks and a pawn in exchange for a queen—a slight material gain; and meanwhile he has obliterated White’s mating threat and reached a winning position.
It's yet another study in the importance of examining every possible check, both on the current board and on the board as it would look after an exchange or two.