This is Black's idea.
1. d2d4  1... d7d5  2. c2c4  2... d5c4  3. g1f3  3... g8f6  4. e2e3  4... e7e6  5. f1c4  5... c7c5  6. e1g1  6... b8c6  7. b1c3  7... a7a6  8. d4d5 A critical move and only the sixth most played. The game now enters tactical territory that, judging by the quality and speed of play, was known to both parties. 8... f6d5 An accurate move. 9. c3d5 