offramp: Petrosian was only about 17-years-old when he played this game against study composer Kasparyan, who was 36.
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Black is threatening 29...Ne2, so Kasparyan plays 29.Bd1. But Petrosian plays 29...Bh6, with two threats: ...Bxc1 and ...Be3, both of which win the exchange. But even after losing the exchange Kasparyan carries on because Black's King position looks very airy. White puts lots of obstacles in Black's way and manages to create a passed pawn. In order to keep that passed pawn he offers up his pawn on b2. 
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37...Qxb2.
Oddly, the crucial part of this very good game comes right on the last move.
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47.Qb4. Petrosian now played 47...Re6, and white would now take the pawn with 48.Rxc4
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...leaving a roughly level game; the R + B on b7/a7 cancel each other out. But if you look closely you'll see that 47.Qb4 does not threaten 48.Rxc4, because Black would play 48....Qa1+, then 49...Qe5+ and 50...Re1+
click for larger view...which wins straight away. So after 47.Qb4
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black has a breathing space to play any move. Any move that DOES NOT interfere with a subsequent 50...Re1+! Unfortunately, 47...Re6 DOES stop a subsequent ...Re1+, because after ...Qa1+ and ...Qe5+ the queen is in the way of the rook and the game is drawn as before. It's almost like a study.
What should Petrosian have played at move 47? I dunno. ...Kf7, perhaps? | 
Kibitzing
