Under the cover of the Ne5, Black quietly prepares a decisive attack. White has nothing with which to oppose it.

White is suffering, so he might as well have something to suffer for! More practical chances were offered by 31.Bxa4, although after 31...g4 (Black also has a dangerous attack after 31...f3) 32.Bd1 Kh7!, with the threat of 33...f3, Black should win in any case.

With the help of a tactical operation, White simplifies the position, and even obtains some material advantage.
In a ‘normal’ course of events (26.Nxf6 Nxf6), Black retains a slight advantage, due to White’s weak pawn structure on the queenside.

Although White has obtained two knights for the black rook, his knights are positioned on the edge of the board. Black can attack the weak white pawns on the queenside. The position remains double-edged.

Black was counting on the fact that the short-range knights find it difficult to fight against an outside passed pawn. But Black had missed something in his calculation of concrete variations.
Correct was 34...Ke5! 35.Nfe4 Rxb3 36.Nf3+ Kd5 37.Nf6+ Kd6, which allows Black to continue the battle, probably with a drawn outcome.

Black’s problem is that his queen is being attacked with tempi, and the solution is a radical one:

A quiet course of the game would promise Black some unpleasantness:
A) 19...Qa5 20.Qb5 Qxb5 21.Bxb5 +/=
B) 19...Qc5 20.Qb5 Qf2 (20...Qxb5 21.Bxb5+/=) 21.Ne4 Qh4 22.g3+/= – the black queen is banished to h6, which guarantees White the initiative.

A good decision.
Accepting the sacrifice with 20.Nf6+ exf6 21.Rxd8 Rxd8 22.exf6 Bxf6³ leads to a position where the white king is in serious danger, whilst the black pieces are well coordinated.

Black has satisfactorily resolved the problems he faced and has developed his pieces. The game is equal.