This natural move is a novelty. The rook is  entering the battle while at the same time it overprotects the center.
1. d2d4  1... g8f6  2. c2c4  2... e7e6  3. g1f3  3... d7d5  4. g2g3  4... f8b4  5. b1d2  5... e8g8  6. f1g2  6... b7b6  7. e1g1  7... c8b7  8. f3e5  8... a7a5  9. d1c2  9... a5a4 