1. c4 g6 2. Nf3 Bg7 3. d4 Nf6 4. Nc3 O-O 5. e4 d6 6. Be2 e5 7. O-O Nc6 8. d5 Ne7 9. Nd2 This is well known position from King's Indian. Here black has many options: 9Bh6, 9Nd7, 9Ne8, all moves with idea to prepare and execute f5 break in the center. But all those moves, although active, appear somehow as "passive" because it gives white initiative on queenside by playing 10. b4! That is why Fischer decided to close the center and to get an attack on white king. So he played:
9... c5! It is clear now that white will play Ne8 or Nd7 next move followed by f5, so white needs to play active on queenside preparing and executing b4 break!
10. Rb1 Larsen wants to play 11. b4 direct, without a3 move.
10... Ne8 11. b4 b6 12. a4 f5 13. a5 This is typical idea in this kind of pawns structure. The move a5 is designed to clear a4 square for queen or knight, depending on circumstances, and to open some lines on queenside, sector where white stands better. Meanwhile black has to concentrate on kingside and to find a way to attack.
13... Nf6 14. Qa4 Bd7 15. Qa3 Bh6! A very good move. Black wants to exchange his inactive bishop on g7 for his mate on c1, which also will leave white with dark square weaknesses, especially f4.
16. Bd3 Instead of 16. Bd3 better was maybe 16. bxc5 bxc5 17. a6! and white creates some counter-chances on queenside.
16... Qc7 17. bxc5 bxc5 18. exf5 Even here it wasn't late to play 18. a6
or 18. Nf3 Bxc1 19. Rfxc1 with dynamically equal game
18... gxf5 Let's take a look at this position. By taking on f5 white gave a very strong center to black, e-f phalanx controlling very important central squares d4, e4, f4, g4; also it opened g file which is clearly in black's favor. Of course, this idea of exchanging on f5 is possible and pretty common in King's Indian, but usually it is followed by move f4 for white trying to limit or stop as much as possible the phalanx. Usually white's plan is like this: after exf5 gxf5 f4 e4 Bc2(e2) Kh8 white is playing Nd1! That knight will go on e3 blocking e passer, protecting g2, attacking f5 weakness and supporting g4 break after a previous preparation with Kh1 and Rg1. But of course, here white can't play f4 so that's why I think move 18.exf5 was a mistake.
19. Bc2? A rather dubious move. Better was 19. a6 with Rb7 to come or
19. Nb5 Bxb5 20. Rxb5 followed by 21. Nb1 Bxc1 Rxc1 with playable position, although in all lines black keeps the initiative.
19... a6! Great move!! This simple move stops 20. Nb5 idea and also 20. a6 for white.
20. Nde4 Bxc1 21. Nxf6+ Rxf6 22. Rfxc1 Raf8 23. Rb6 Bc8 24. Ne2? We can see clearly that black has no counter-chance on queenside while all black pieces are ready to jump on white's king neck. With his last move 24. Ne2 white leaves e4 square a bit without control, that's why black immediately reacts in the center with:
24... f4! A very good move which gives life to bishop on c8, frees f5 square for black pieces and also it threats f3 opening up white king.
25. Be4 Nf5 26. Rc6 Qg7! Amazing how Fischer plays. All his pieces are coming on kingside where main battle will take place
27. Rb1 Nh4! 28. Qd3 The threat is obviously to play 28f3 with crushing attack. A very important line is: 28. Rxc8 Rxc8 29. Qh3 attacking both the rook on c8 and knight on h4 and it looks like white is even better. Problem is that after 29. Qh3 black calmly replies with 29... Rcf8 since white can't really take the knight on h4 because of the move 30...Rh6 trapping queen :)
28... Bf5! Black is trying to exchange the defender of light square.
29. Kh1 f3 30. Ng3 fxg2+ 31. Kg1 Bxe4 32. Qxe4 Nf3+ 33. Kxg2 Nd2 and white is losing an exchange and not only that (f2 is a goner also), so that's why he resigned.