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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.