The key move is Red’s R2=6, threatening C9+3 followed by R6+1. If Black tries to avoid smothered checkmate by playing A4+5, then comes the Iron Bolt checkmate after Red’s C9=5.
The critical move is #5/11 offering the Chariot the second time. R5=6 instead would be a fatal mistake, because Black could capture the Cannon with check, and turn the table around!
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