Black is threatened with the loss of a rook on his back rank. Meanwhile he has an offensive opening of his own at the other end of the board: his knight masks the bishop on c5; since the bishop would attack White’s king if unmasked, the key question is what damage the f2 knight might do while Black fends off that check. Do you see how this offensive opportunity relates to the threat of RxRe8 that currently worries Black? The strength of White’s threat depends on the coordination of his rooks on the d-file; that is what prevents Black from being the one to gain with RxR. Things would be different if the connection between White's rooks were disturbed—and as it happens, Black can cut the rooks off from each other and discover check at the same time with 1. …Nd3+, 2. Kh1, RxR+. Black wins a rook; the knight severs the line of protection to White's d8 rook—and then the knight is protected against capture by the Black rook that moves onto d8.

The more obvious 1. …Ne4+ doesn't work. White again replies 2. Kh1, so now Black can play NxR, perhaps thinking he has won the exchange after White recaptures; but White doesn't recapture right away. He instead executes that ongoing threat of RxR+. The priority of check means that he holds the initiative while Black replies Bf8 (forced); and now White still has the recapture BxN waiting for him at the near end of the board.