Rama: Both Larsen and Spassky were hot in this tournament.
We've seen that Larsen's use of the Bird's was strategical and purposeful. 4. b4 ..., sets up the coming Q-side action. 7. ... c6, is necessary to forestall the advance b4-b5, and then 8. a4 ..., threatens a kind of Minority Attack.
11. d4 ..., blocks the Bb2, but it does much worse things to the black position. At first I did not like 11. ... Ne4, but since ... f5, is always in reserve I can see that the pawn e4 is not all that weak.
The position after 15. Qd1 ..., looks like something that Larsen has prepared in advance. The existence of the move allows the coming 16. c4 ....
The follow-up moves 17. b5 ..., and 19. bxc6 ..., present black with a crisis. Ordinary tactics like 19. ... Nxe3, followed by 20. ... Qxe5, do not work because of 20. cxb7! ..., with the Rb1 ready to support. Spassky's 19. ... b6, is ingenious in that it preserves his Q-side pawn structure, and after 21. ... Qxc6, he regains the material.
22. d5 ..., starts a whole new phase of the game. Somehow, Larsen has spied the e6 square and determines to put his Knight there no matter what the cost. With 24. Nd4 ..., he has it in hand but first makes a series of preparatory moves. When it comes on 31. Ne6 ..., black is reduced to nearly complete passivity. The sequence ending with 35. Nxg5+ ..., shows this clearly.
36. c5! ..., is a master-move. No matter what, white gains access to the checking square a4, and then access to all the white squares in the vicinity of the enemy King.
Black's passivity is now complete. In Zugzwang, he proceeds to wreck his position with pawn advances since his pieces cannot move. White waits until move 52 to play Qe6! which forces the Q-exchange. The threat 54. Nh7 mate, is the point which allows 54. Nxe4 ..., and 55. Nxc3.
But now it is Boris who cooperates with 55. ... Ne4?, which is surely a blunder. After the exchanges, white's pawns regain their mobility and in just three moves it is over -- the e-pawn is going to queen.
Does the presence of 55. ...Rd8, nullify Larsen's masterpiece? Only for chess-theorists. Larsen created the conditions that produced the blunder in the critical game, which in itself is chess of the highest order. Remember, Spassky had obtained 8 points in his last 10 games in this tournament; he was very very tough to beat at this moment.