Notice the Almost-solutions

In the given position, the critical crossroads lies on the square d6 🚦 .
Horizontally – it is part of the potential danger line between the ♜ and the ♔.
Vertically – it’s the path for the ♖ from the 7th rank into the heart of the board.
Diagonally – it’s on the line of sight and attack of the ♗.
And meeting them all there is also the ♘, for which d6 is one of its two target squares for attacking the ♚.
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We said “one of its two targets” – so let’s talk about the other: f6.
If the ♘ were to reach that square (taking the place of the now-departed ♟), it would immediately trigger an undesirable response for White: ♜xf6+. And if the ♘ leaves the ♟ in place, then in turn Black could move it away from f6 giving check (via the ♜).

Thus, a natural plan suggests itself: on the first move, let’s block the ♜’s path to the right. This both prevents check on the ♔ and prepares the second move ♘xf6#.

🎖️🎖️ There are two volunteers ready to perform the block on d6 as the first move – the ♗ or the ♖.
However, whichever of them arrives there will also block its own partner (not just the ♜), and each of these White self-blocks allows Black a defense by moving a♟ and freeing an escape square for the ♚ (d4 / f4).
🔀 This is a White Grimshaw 🔄 .

🗝️ The actual winning key is achieved a bit differently: preemptively blocking the ♟ on f6 so it cannot advance and cause trouble, while at the same time preparing a different mating threat – ♖xf4# 🏆 .