White plays with his checks and sees that 1. Re8+ calls for Black to interpose with Rf8, with his king then guarding against RxR. When you see an enemy king doing defensive work like this on its back rank, you know that a check may force it to abandon its defensive role and move back toward the corner. The check can occur midway during a sequence rather than at the start. Thus White follows 1. Re8+, Rf8 with Bd5+, which requires Black to play Kh8: notice that when Black interposed his rook he also walked it into a pin of its own so that it can’t interpose. Now White has RxR#.