12. Be3 looks strong due to the threat 13. dxe5 and since Black can't take Qxb2 (13. Rfb1 would win the Queen). But after 12. Ng4! White is in a dire situation; at least he must part with the bishop pair. If, after 13. a5 Qc7, he plays 15. Bd2 to preserve the two bishops, he loses a piece by exd4! 16. Qxd4 d5! because of the mating threat at h2.

After 16. dxe5, White probably expected either dxe5 or Nxe5 or Nxe3, but surely not the totally surprising 16. ... d5!! After 17. exd5 Qxe5 White can stop the mating threat only by the weakening moves 18. g3 Qh4 19. h4. After 19. ... Bxh4, White dare not take the bishop, probably because of 20. gxh4 Ne5! (Qxh4 or Bf5 don't lead to anything) 21. Qe4 (anything else loses quickly) Nf3+ 22. Kg2 (Kh1 Bg4! and there is no defence) Nh4+ 23. Kh2 (Kg1 or h1 lose even faster) Nf3+ 24. Kg3 Qh3+ 25. Kf4 Qg4#. But - and this was most difficult to see OTB - 23. Kg3!! would have held the position, there is no win, and Black must take the draw after Nf5+ Kg2 Nh4+ etc. (Of course, I haven't found this myself, my silicon friend has made me aware of that possibility.)

After 18. Kg2, the text move Ne5 is not the strongest possibility. Nxe3 would have been stronger, since 19. Qxe3 Bg5! followed by Bh3+ wins the exchange, and 19. fxe3 Qg4 gives Black good pressure.

After the weaker 18. ... Ne5, White succeeds in fighting himself back into the game. He plays extraordinary well till move 24. Unfortunately, he overlooks the strong possibility 25. Qxf6!!, which would have given him an approximately even game (gxf6 26. Ne7+ Kh8 27. Nxg6 fxg6 28. b3, and the danger is over).

After 25. Nxf6+, Black can keep the queens on the board, with a dangerous attack. The threats are Bb7 and Bxb2, hence White plays 26. Qc3.

28. Bc5 is necessary to stop Rxc2+, and White probably thinks he can support his Bc5 by b4 if necessary (and get out of the pin by time). Indeed, after e.g. the immediate Qf5 29. b4 a5 would not be possible since the white rook controls a5. This explains Black's manoever 28. Rfe1, threatening Re2+ so that White has to dispute the e-file by Re1 (other moves are no better), and after exchanging rooks White does not longer control a5, so that Black can exploit the pin by 30. ... Qf5 31. b4 a5, with a winning position in any case.

34. Bxb4 would have been necessary then to immediately remove the dangerous passed pawn, but after Rc2+ followed by Bxf3, Black's much better off, too. (White played 34. Bxa7 to be able to interpose at f2.)

A nice final twist is that White could play 41. Rxb2 Rxb2 42. Bd4+ if his king already stood on g3 so that the Bf2 weren't pinned ... as so often, one single tempo is decisive in the end game.