[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?
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.