The point is that 39 x does not work due to 39 ... g6+! 40 x d1=+!, this zwischenschach (<in-between check>) winning easily after 41 b2 px. The zwischenschach 40 ... d1=+! is what Sadvakasov missed when he played 38 g3?.

(24...Bxf6 25.Bf4 Be5 26.Bxe5 is not better for Black) ---------------- - - - - - - - - - -
31.Nxc8+ Rxc8 32.Rxc8 Qxc8 since d4 is protected by N/f5, which in turn is guarded by Q/c8.

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35...Ka7-a8! was necessary to avoid 36.Nc6+ <double check!> Ka8 37.Qa7#

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After 36.Qg3 Qg6 is best (with 37...d2+ threatened and White cannot reply 38.Qxg6 since ...d1=Q#). So 37.Qxg6 hxg6 gives Black the endgame advantage. 38.Nxe6 fxe6 39.Kc1 e5 40.Kd2 e4 wins .

The following variations show how easy Black can err.

36...Qh6 loses d3-pawn

36...Qd2 37.Qg8+ Ka7 38.Qg1! sets up mate threat as before.

36...Qxg3 37.hxg3 Ka7 <37...Bg4 38.Kc1 and Black could lose the d-pawn)> 38.b3 Kb6 39.Kc1 again.