This incredible idea is only White's 16th most popular choice in this position $1 This shows that it's still possible to find fresh ideas even in the most studied openings. But what's the idea of this mysterious move $2 From what I understand, the reasoning is this: 9.Nc3 is premature because Black can take the c4-pawn and then defend it with b7-b5. That's why White makes an useful move and intends to answer 9...b6 (the most natural move) with 10.Nc3. If Black captures the c4-pawn and plays b6-b5 in this case, then White has an extra tempo and the pawn is quite useful on h4.
1. d2d4  1... g8f6  2. c2c4  2... e7e6  3. g1f3  3... d7d5  4. g2g3 The Catalan has become a powerful weapon for Nepomniachtchi. After all the work he had to do in order to play it with Black in the match against Carlsen, no wonder he decided to try it with the other color. 4... f8b4 This check is one of the most reliable options for Black. 5. c1d2  5... b4e7  6. f1g2  6... e8g8  7. e1g1  7... b8d7 The point is that the bishop is not well placed on d2 in the lines where Black refrains from taking on c4. With the bishop on c1, White can play b3-Bb2 or Qc2-Nbd2, plans that are not available now. 8. d1c2  8... c7c6 