Not the best move for Black. Ignoring the threat of White's position, they grab the e-pawn. This initially seems like a good move because Black has 5...d5 after 5. Nxe4? forking the knight and bishop.

However, White can now capture Black's unprotected f-pawn with check: 5. Bxf7+ Ke7 6. d4! and Black cannot yet take on g5 without losing his queen.

5. Nxf7 is a mistake, as after 5... Qh4 Black gains an advantage even against White's best defense.

1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.Ng5 Nxe4

Wikipedia has related information at Two Knights Defence.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.

• Nunn's Chess Openings. 1999. John Nunn (Editor), Graham Burgess, John Emms, Joe Gallagher. ISBN 1-8574-4221-0.