comment: A subtle move to gain access to the e4-square.
context: 1. f2f4 The most natural move, opening the diagonal for the queen and attacking a7. 1... a8b6 A peculiarity of FreeStyle Chess: it's possible that Black's first move is already clearly better than the alternatives, as is the case here. 2. b1c3 Black has to be careful with the move Nc3-b5, attacking a7 and c7. 2... f7f5 Now White shouldn't play 3.Nb5 as Black can take on a2. 3. a1b3  3... b8a6 Caruana keeps the c7-pawn protected. 4. d2d3  4... c7c6 Forever preventing Nb5, but the position of the knight on b6 is weakened. 5. c1e3 Immediately exploiting the problems with Black's last move. 5... g7g5 The best chance to create counterplay. Caruana undoubtedly had prepared this move in advance. 6. f4g5 Carlsen accepts the pawn sac. 6... e7e5 The idea with the sac is to control the center with the pawns. Next is d7-d5.