DrMAL: Position transposed into Accelerated Dragon with against Maróczy bind many openings do, typical Sicilian version is 1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 g6 5.c4 Bg7 where 6.Be3 is typical move and 6...Nh6 (instead of 6...Nf6) leads to sharp position with pawn break f5 to strike at bind. 9.exf5 is required if white wants any kind of advantage. Usual move is 10.Bxd4 to get bishops advantage but 10.Bxh6 allows white DSB to have role on h6 for
clearer edge, here is computer eval.
Houdini_20_x64:27/70 1:54:19 36,559,899,975
+0.32 10.Bxh6 Rxf5 11.0-0 d6 12.Rc1 Rf7 13.Bf3 Be5
+0.12 10.Bxd4 Nxf5 11.Bc5 Qa5 12.Ba3 d6 13.0-0 Be6
12...Qa5 transposes with 12...Be6 in computer line but Spassky played 12...Bfd4 to grab the outpost this was also excellent move. Game is basically equal, white's arguable edge can only come if B on a3 can do something, so white played to free it, while Spassky continued to disrupt white position, taking advantage of B not on h6 to build on f-file.
17...e5 was basically forced, black was not about to give up strong N position. Now white wanted to bother other N with P advance b4-b5 but this plan is simply too slow. After 18.b4?! Spassky could have played 18...Nxe2 also strong, but he preferred to keep pieces with 18...Be6! starting brilliant attack, black already may have edge here. Knowing 18...Nxe2 was also strong white white played 19.Bd3?! using up one critical tempo. 19.b5 was better but best move was 19.Ne3 very subtle.
Timing was just right for 19...Bg4! Spassky's ability to understand ideal tempo and initiative was key factor to his becoming WC. Game is not so well known and underrated, it was one of Spassky's most brilliant attacks. Computer evaluates 20.Qd2 as slightly better but 20.f3 did not fully anticipate 20...Bxf3! other sac 20...Nxf3+ was strong giving advantage but this move nearly wins game already.
Houdini_20_x64: 28/84 36:03 25,172,740,934
+1.14 20. ... Bxf3 21.gxf3 Nxf3+ 22.Kh1 Qh3
+0.34 20. ... Nxf3+ 21.gxf3 Bxf3 22.Qe1 Nxb4
Furhman played the best he could, for example, 22.Rxf3 leads to destruction after 22...Rxf3 23.Be2 (forced) 24...Rf2 24.Qe1 (forced) 24...Rc3 25.Rc3 (forced) and simply 25...Kg7 to dispel idea of fork on e7, white's position is basically resignable here, if game goes into endgame R+P+P is technical win against white's pair of bishops, easy for player like Spassky.
23.Qe2 was required to stay alive (23.Qc2 loses to 23...Ne1 as well 24.Rxf7 Nxc2 25.Rxf8+ Kxf8 26.Bxc2 Qf3+! 27.Kg1 Qg4+ with Qxc4 and black queen all over white's timid position. I guess Furhman did not realize 23...Ne1! obliterates any shadow of hope. Game here is one Fischer would have put in book if he played, it is similar in style, and it shows not only dangerous opening weapon of Nh6-f5 plan but also extreme brilliance in understanding very subtle timing and proper risk behind sac.