This seemingly primitive move, with the unmistakable intention of advancing the pawn to g4, has become a dangerous weapon in the hands of many grandmasters. It makes sense; after all, why worry about not being able to short-castle if White wants to take the king to the other side $2
1. e2e4  1... c7c5  2. g1f3  2... d7d6  3. d2d4  3... c5d4  4. f3d4  4... g8f6  5. b1c3  5... a7a6 