Figure[Black to move]

See how the current idea works here. White’s king looks secure; it is protected by its rook. If Black drops a rook or queen onto the first rank, White can either capture it with his rook or leave his rook where it is and perhaps create a flight square by moving his h-pawn forward. But Black can change all this with a sacrifice on the square in front of White’s rook: Qxf2+. This threatens QxR# with cover from the rook on f8, so White has to play Rxf2. Now White’s back rank is bereft of defenders. True, White can return his rook there, and he indeed must do so in reply to Black’s next move: Ra1+; but once White replies Rf1, he is in the same position Black suffered in the previous frame. Here Black has two rooks with clean lines to the square next to White’s king, with one able to provide protection for the other; so Black plays RxR with either rook and mates.