Lesson
White to move. Main line: 1. e4 e5 2. Nf3 Nc6 3. Bc4 Nd4 4. Nxe5 Qg5 5. Nxf7 Qxg2 6. Rf1 Qxe4+ 7. Be2 Nf3#
After 4.Nxe5?? Qg5! forks the knight and g2. White’s "best" try 5.Nxf7 grabs a second pawn and forks queen and rook — and loses by force: 5...Qxg2 6.Rf1 Qxe4+ and the bishop must block on e2, pinning itself.
7...Nf3# is the picture worth memorizing: the e2-bishop is pinned by the e4-queen so it cannot capture the knight, d1 is blocked by White’s own queen, d2 and g1 are covered by the knight, and the g2-pawn that would have taken on f3 is gone.
The honest lesson: 3...Nd4 breaks opening principles (moving the same piece twice, ignoring development) and 4.Nxd4 refutes it calmly. Traps are bets on the opponent’s greed — this page shows both the payoff and the refusal.
The refutation: 4.Nxd4: 4. Nxd4 exd4 — simply trading knights leaves White comfortably better. The trap only bites the greedy.