While 2...e6 is played to prevent White's e4, the move 2...g6 allows it. Black hopes that the Bishop on the a1-h8 diagonal will offer good counterplay against White's strong center. After 3.Nc3, Black has two good systems.
The first is 3...Bg7, allowing White to continue unimpeded central development. The diagram shows a typical position.
Black played ...d6 to prevent White from advancing e5, which would chase the Knight on f6 away from the center. Both players will castle Kingside and then address the development of the Queenside.
Instead of ...d6, Black can challenge White's central buildup with an immediate 3...d5. If White continues to pursue e4, the following position can occur.
1.d4 Nf6 2.c4 g6 3.Nc3 d5 4.cxd5 Nxd5 5.e4 Nxc3 6.bxc3 Bg7
White has a strong center, but is weak on the long a1-h8 diagonal. Black will challenge the center and exploit the pressure on the diagonal by playing ...c5.
