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Why it works
After 6.Nc3 Black tries to repeat the 'pin trick' from the main trap and plays 6...Bb4??. But now White's bishop on d2 is free (unpinned), and White has Rb1 with double threats on the b-file. Black slides into a forced sequence: knight sacrifice on e6 to open the line, then a second knight on f6 to weaken the king. The end comes from the rook on d8 — Black's king is trapped between its own f8 bishop and the attacking white forces.
Refutation
6...Qb4 — best try, retreat the queen instead of moving the bishop. Black is still down a pawn objectively but avoids the forced mate.