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d c b 8 a ←→ to step through moves · Home / End — first / last Source
Why it works
From's Gambit gives Black a raging dark-square attack for a pawn, and 4...g5! looks like it merely threatens ...g4. White plays 5.h3?? to stop it — and resigns by force: the h-pawn was the only defender of g3 (any ...Bg3+ was met by hxg3). With the pawn on h3, 5...Bg3# is immediate mate — the f2-square is empty because the f-pawn left on move one, so the bishop hits e1 with nothing to block, capture or run to.
Refutation
5.d4! ignores the demonstration and takes the centre — after ...g4 White has Ne5/Ng5 squares and stays slightly better (engine +0.55). 5.g3 is the other sound setup. The one thing White cannot afford is touching the h-pawn: it is chained to g3 for the rest of the opening.