Anastasia's Mate

Look at the black king on g8: its own pawns on f7 and g7 already box it in from the front, and the rook on f8 blocks f8. White's knight on c6 can jump to e7, where it covers g8 and g6. After ♘︎e7+ the king has to step onto the h-file, and White's rook on f3 slides to h3 to meet it there — the king cannot come back to g8 or g6 because of the knight, and g7 is its own pawn, so the check is mate.

Which knight check also covers g6 and g8?

♘︎e7+ — The knight checks from e7 and takes g8 and g6 away at the same time. The king has to step onto the h-file — h7 or h8 — and wherever it goes, ♖︎h3 is checkmate: the rook covers the whole h-file, g7 is blocked by Black's own pawn, and the knight guards g6 and g8.

Slide the rook to the file the king stands on.

♖︎h3# — Checkmate. The rook attacks h7 along the file; the king cannot go to g8 or g6 because of the knight on e7, and g7 is its own pawn. Two pieces against a king walled in by its own pawns.

The same shape can appear anywhere on the edge — here the black king has wandered to g3, right next to White's rook on f2 and knight on f4. A knight on e4 would cover g3 and g5, so after ♘︎e4+ the king has one square, h4. On h4 the h-file is Anastasia's file: the rook goes to h2 and the king's own pawn on g4 blocks the last exit.

A knight check that leaves the king a single square.

♘︎e4+ checks and covers g3 and g5. Black's king has exactly one square, h4 — and there ♖︎h2 is checkmate: the rook covers the h-file, the knight on e4 takes g3 and g5, and the pawn on g4 is Black's own piece blocking the last square.

The rook to the edge file.

♖︎h2# — Checkmate — the same shape turned on its side: a knight on e4 taking g3 and g5, the king's own pawn on g4 in the way, and a rook on the edge file.

Black's rooks on b2 and c2 are one move from attacking White's king on a1, so White has to mate first. The knight on e6 already covers g7 and g5, and the rook on d8 owns the back rank. Which move starts the mate?

What makes Anastasia's mate work?