White has many options but decides to exchange the bishop for the knight, which will more or less force Black to do the same.
1. d2d4  1... g8f6  2. c2c4  2... e7e6  3. g2g3  3... d7d5  4. f1g2  4... b8c6  5. g1f3  5... a8b8 A very rare move. According to my database, it is just the 10th choice for Black in this position. Caruana usually shows interesting ideas in the opening. 6. c4d5  6... f8b4 Already a new move. 7. b1c3  7... e6d5 