Giving meaning to the pawn on a3.
1. c2c4  1... c7c5  2. b1c3  2... b8c6  3. e2e3  3... g8f6  4. d2d4  4... d7d5  5. g1f3  5... e7e6  6. a2a3 White decides not to resolve the central tension and makes a waiting move. This is the most-played move in this position. 6... a7a6 Black follows the same strategy. 7. d4c5  7... f8c5 