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e f g 1 h Why it works
White has just handed over a knight for a pawn and dragged the king to f7: 6.Nxf7 Kxf7 7.Nf3. Black is a piece up and wants a developing move that also does something, and 7...Bg4 is exactly that move in every other opening — it pins the knight, it grabs an active square, it looks like the start of consolidation. Here the pin is imaginary. 8.Ne5+ arrives with check, so nothing is pinned: the king has to step aside, and the moment it does the bishop on g4 is attacked twice and cannot be held. 9.Nxg4 Nxg4 10.Qxg4 restores the material with the black king stranded, and 11.Bc4+ Kh7 12.Bd3+ Kg8 13.Qe6# finishes it. 375,000 games have reached this position. Under 1000, 40% of players walk into it; at 2500, 1.8% do.
Refutation
7...c5! is what holds up in practice: Black hits the centre before White finishes developing, and White's score drops from 82% to 47%. The rule behind it — while the king sits on f7, a pin on the d1–h5 diagonal lasts only until White finds a check. Any developing move that allows Ne5+ with tempo gives the extra piece straight back.