4.0-0? exd4 5.Nxd4 Qb6 Black wins a piece
4.dxe5 d4 , followed by ...Nxe5
4.dxc5 d4 , followed by ...Nxc5, also favours him.
4.dxe5 d4 , followed by ...Nxe5
4.dxc5 d4 , followed by ...Nxc5, also favours him.
Initiating an attack on the queenside, which indirectly threatens the safety of the white king.
White should have restricted the enemy knight by 10.f5 , although Black maintains the initiative after d5.