White's pawn on d5 is now defended and attacked twice. After 5... Nf6 there is no good way for white to defend the pawn a third time and it will fall. 5... Bf5 seems like a good move as it forces White's bishop to move again with 6. Bb3, but after 6... Nf6 7. Bg5 Nbxd5 8. Nxd5 Nxd5 9. Qf3 Be6 Black's game is worse.