How did Timman decide on d5 in this position? In Gheorghiu-Timman 1982, how did Timman decide to play 34…d5? It seems like an amazing move to find.

 [FEN "2r5/p1r2b2/1p1p1k1p/PP2ppp1/2P2P2/3KP1P1/2RN3P/2R5 b - - 0 1"]

 1... d5
In this position, black clearly is better with heavy pressure down the c-file, but it is clear that c4 is also well defended, and that black cannot bring any additional pressure on c4 except with d5. So, can black do anything else like try and infiltrate with Kg6-h5? I ran some logical lines with Stockfish, but they are labyrinth-like, and while it is clear that black is still the one pushing, nothing was clear. In those lines, black also has to be on the lookout for possible e4 attempts by white, and also axb6 axb6, Ra1-a6 lines that can gain great counterplay should black win b6, and start pushing the b5 pawn.

d5 is a way to increase the advantage, and clarify the position. Timman also saw that the white knight was no match for the good bishop, and better black king position. Gheorgiu should have avoided that at all costs.

 [Event "Olympiad-24"]
 [Site "Valetta"]
 [Date "1980.11.25"]
 [Round "5"]
 [White "Gheorghiu, Florin"]
 [Black "Timman, Jan H"]
 [Result "0-1"]
 [ECO "E32"]
 [WhiteElo "2605"]
 [BlackElo "2600"]
 [PlyCount "106"]
 [EventDate "1980.11.20"]
 [EventType "team-swiss"]
 [EventRounds "14"]
 [EventCountry "MLT"]
 [WhiteTeam "Romania"]
 [BlackTeam "Netherlands"]
 [WhiteTeamCountry "ROU"]
 [BlackTeamCountry "NED"]
 [FEN ""]

 1. d4 Nf6 2. c4 e6 3. Nc3 Bb4 4. Qc2 O-O 5. a3 Bxc3+ 6. Qxc3 b6 7. Bg5 Bb7 8. e3 d6 9. f3 Nbd7 10. Bd3 c5 11. Ne2 Rc8 12. O-O Ba6 13. b4 cxd4 14. Qxd4 h6 15. Bxf6 Qxf6 16. Qxf6 Nxf6 17. Rac1 Nd7 18. Rfd1 Ne5 19. b5 Bb7 20. f4 Nxd3 21. Rxd3 Rfd8 22. Rdc3 Rc5 23. Nd4 Rdc8 24. Nb3 R5c7 25. a4 Bd5 26. Nd2 f5 27. Kf2 Kf7 28. g3 Ke7 29. Ke2 e5 30. Kd3 Be6 31. a5 g5 32. Nb3 Kf6 33. R3c2 Bf7 34. Nd2 d5 (34... Kg6 35. axb6 axb6 36. Ra1 $11) 35. fxe5+ (35. axb6 axb6 36. Ra2 $2 dxc4+ 37. Kc3 Rc5 {And black is too fast.}) 35... Kxe5 36. cxd5 $2 {Going into the bishop versus knight ending was clearly hopeless.} (36. axb6 $1 axb6 37. c5 $1 Rxc5 (37... bxc5 38. Nf3+ Kf6 39. Kd2 Bh5 40. Nd4 Ke5 41. Nc6+ {And if black hopes to win, he will need to sacrifice the exchange here.}) 38. Rxc5 Rxc5 39. Rxc5 bxc5 40. Kc3 Kd6 41. Nb3 c4 42. Nd4 Kc5 (42... Bg6 43. Kb4) 43. Nxf5 $11 {This leads to an ending that is possibly holding, but would have been very hard to find at the board.}) 36... Rxc2 37. Rxc2 Rxc2 38. Kxc2 Kxd5 39. axb6 axb6 40. Kc3 Bh5 41. Nb3 Be8 42. Kb4 Ke4 43. Nd2+ Kxe3 44. Nc4+ Ke2 45. Nxb6 f4 46. gxf4 gxf4 47. Nd5 f3 48. Nc3+ Ke1 49. Ne4 f2 50. Nxf2 Kxf2 51. Kc5 Bd7 52. b6 Bc8 53. Kd6 h5 0-1