How is c4 the best move in this position?

Question

[FEN "rn2r1k1/pp2qpp1/3b1nbp/2pp4/6PP/1B1PBP2/PPPQNK2/RN2R3 b KQkq - 0 14"]

14... c4 15. dxc4 Nxg4+

I set up the above position in Stockfish and the initial analysis was that c4 was the best move. After some time it changed its evaluation and gave Nc6

I am trying to understand the idea behind the continuation C4, It looks like a pawn loss and a waste of tempo, If the plan is to sacrifice the Knight, why not just sacrifice it at move 14?(move 1 in the above position?) instead of wasting the pawn then "sac" the Knight?

Answer 1

The position is very advantageous for Black anyway, but there are two points in including 14...c4 15.dc4 before sacrificing the knight:

• Deflecting the wPd3 give access to e4 to Black pieces if White plays fg4. Especially the Bg6 will gladly join the mating attack.

• Since Black Pd5 will be undefended after the knight departure, it is worth preventing any Bxd5 by White : the bishop there would be excellently placed in the center, defending h1/g2/f3 on one diagonal and attacking f7 on the other. A White pawn on d5 (after a possible cxd5) will be much less annoying, threatening nothing and blocking the Bb3.

• At any moment, Black can drag the white Bishop to the unprotected square c4, where it might become a target to horizontal attacks by the queen. E.g., if 14...c4 15.dc4 Nxg4 16.fg4 Qh4 17.Kf1 Qh3 [simply 17...Be4 must already be winning] 18.Kf2 Qh2 19.Kf1, then 19...dc4 20.Bxc4 Qh3 21.Kf2 Qxg4 is a pattern to take into account even if it also exposes the Bd6 on the d-file.