The defender becomes the attacker.

The immediate 7.g4 is a rude answer to black's trustowrthy O'Kelly defense. But is it correct ? Probably not, but that wont keep me from winning with it.

1.d4 d5 2.e4 dxe4 3.Nc3 Nf6 4.f3 exf3 5.Nxf3 c6 6.Bc4 Bf5
The O'kelly, which can lead to the Ziegler defense, but white has other plans.

7.g4 !!! (diagram)

7...Nxg4 8.Nh4 g6 9.Nxf5 gxf5 10.h3 Nf6 11.Bg5 e6 12.d5
A key move for the attacker

12...cxd5 13.Bxd5 (diagram)

13...Nbd7 14.Bxb7 Rb8
probably not the best move. 14...Be7 15.Qf3 Rb8 16.Rd1 would give black a small advantage, but it very unlikely any defender would have been able to get this far without mistakes.

15.Bc6 Qb6 (diagram)
best

let's look at the situation : white is a pawn down and his king is defenseless. Black on the other hand has whaethered the storm and is now starting to deploy his forces. So it seems white can safely resign here, isnt it ?

16.Qf3 Be7 17.Rd1 Rd8 18.Ke2

White is desperately trying to get some space as to centralize his pieces

18...h6 19.Bxf6 Bxf6 20.Rxd7 Rxd7 21.Rd1 0-0 22.Rxd7 23.Qxb2 Kd2

White is a piece ahead for two pawns, but it is black who has the safer king and the king. White may be able to survive but it wont be easy. But then again, chances are low that black will be able to get this far against a well prepared white berserker.