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e f g 1 h Why it works
Black tries to hold the c4 pawn — classic beginner greed. After 3...b5 4.a4 c6 5.axb5 cxb5 it looks like Black has kept the extra material, but the a8-h1 diagonal is open. 6.Qf3 targets the rook on a8. Black plays the 'developing' 6...Nc6 to defend b5, missing that the knight itself becomes a target on the long diagonal: 7.Qxc6+! captures the knight with check (through empty d7 and e8). After Bd7 blocks, the queen simply retreats — 8.Qf3, and White is left a piece up for a pawn. The greedy 8.Qxb5?? would drop the queen to 8...Bxb5. The trap dates back to 1604 (Alessandro Salvio).
Refutation
6...Bb7 — the only adequate defense, blocks the a8-h1 diagonal and develops the bishop. After 7.Bxc4 White recovers the pawn, but Black's position has no clear weaknesses. The alternative 6...Qd7 is worse due to 7.Nc3 with threats.