Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bc4/3...Nf6/4. Ng5/4...Nxe4/5. Nxf7

= Two Knights Defence =

4...Nxf7?
White takes with their knight, forking the Black queen and rook. This is inferior to capturing with the bishop because Black can create counterplay with the strong move Qh4, threatening checkmate. Here, White's only defense is 6. Qe2 and Black can force a queen trade with 6... Nd4 7. g3 Nxe2 8. gxh4 Nd4

Even after the White knight finally captures the rook with 9. Nxh8, Black has a fork of their own on c2, and with 10. Ke2, the game continues on without any advantage for White.