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e f g 1 h Why it works
4...Nd7 develops 'flexibly' — and blocks the one square the queen will need. 5.Bxf7+! Kxf7 6.Ng5+ Ke8 7.Ne6! and the queen is walled in by her own army: the d-file is blocked by the knight on d7, c7 by its own pawn, and fxe6 is impossible because the f-pawn died on move five. The queen must flee to a5 or b6, and the e6-knight then feasts — Nxc7+ forking king and rook, or Nxg7+ — recouping the bishop with interest (engine +4.7). One developing move built Black's own prison.
Refutation
4...Nf6 is the move the Pirc wants anyway: it develops towards the centre, covers the g5/e4 complex and — the real point here — leaves d7 free as the queen's escape hatch, so the whole Bxf7+/Ng5+/Ne6 machinery never gets started. The engine calls it a normal opening (−0.5 for Black, standard Pirc territory).