This move looks odd, but sets a trap and is an interesting way to avoid theory. It is a top engine choice and allows Black to blunder 5...Bf5??, which loses a piece after 6. Nxf6+.

The best continuation is 5...Nbd7 6. Nf3 Nxe4 7. Qxe4, which enters a position that is more commonly reached from the Two Knights Attack.

Richard Réti played this move against Savielly Tartakower in 1910. The game continued with 5...e5? 6. dxe5 Qa5+ 7. Bd2 Qxe5 8. O-O-O! Nxe4?? 9. Qd8+!! Kxd8 10. Bg5+ Kc7 11. Bd8#.