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e f g 1 h Why it works
In the 4.Bf4 (Rubinstein) system Black's knight sits on g4 to win back the e5-pawn. After 4...Bc5 5.e3 club players very often reach for the natural developing 5...Nc6?? It is the single most common reply up to 1800, yet it simply hangs the knight. The move 5.e3 vacated e2 and opened the d1-g4 diagonal: 6.Qxg4! wins the knight outright; g4 is undefended and there is no recapture. Black is left a full piece down (on top of the gambit pawn White already holds). The trap recurs because ...Nc6 develops a piece on autopilot and players forget the g4-knight is still loose.
Refutation
Deal with the loose knight first. Correct is 5...d6! (Stockfish's pick): it hits e5 and reopens the ...Ngxe5 route home. After 6.exd6 Bxd6 Black regains the pawn with a normal Budapest game. The rule: never play ...Nc6 while the g4-knight is undefended and the d1-g4 diagonal is open.