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e f g 1 h Why it works
After 4...g6?? Black weakens the a2-g8 diagonal (f7 is now under the bishop on c4) and misses a hidden queen sacrifice. White plays 5.Nxe5! Apparently a blunder: the knight is attacked by the pinned bishop on g4. The greedy 5...Bxd1?? looks winning: Black takes the queen. But it dissolves the pin; the e5 knight is now free, and White forces mate: 6.Bxf7+ Ke7 (Kxf7 is illegal; the bishop is defended by the e5 knight), and 7.Nd5#. The c3 knight jumps to d5 delivering mate. The king on e7 has no escape: d6 (own pawn), d7 (covered by the e5 knight), e6/e8/g8 (bishop on f7), d8 (own queen), f6/f8 (knight on d5 and own bishop). The most famous trap in chess history (18th century, Sire de Légal de Kermeur).
Refutation
5...dxe5! The only correct move. Black simply takes the knight and remains up a piece, the bishop pin on the (now gone) f3-knight having served its purpose. After 5...dxe5 Black is fine: a piece for a pawn, healthy development.