context: 1. c2c4 The most natural move. 1... g8f6 The knight is vulnerable on f6, as we will see in the game. In addition to the potential capture with the bishop on a1, another problem is that White can advance e4-e5, attacking it. 2. g1f3  2... b7b6  3. b2b4  3... c7c5 Caruana isn't afraid of losing the h-pawn. 4. a1e5 A very subtle and interesting move. Keymer wants to close the diagonal before capturing on f6.