Black can try to save the bishop and prevent mate on g6 by playing Red8 30 Nf4 Be8, but after 31 Rfg1 the threat becomes 32 Rg8+ Kg8 32 R1g7 mate. Note that Petrosian saw way in advance (among other things) that black cannot play 31 ... Bh5 to prevent both the rook mate and the Ng6 mate, since the h5 square is covered by the knight on f4. Petrosian was known as a "dull" positional player, but as this game shows, you don't get to that level without having pretty good tactical skills as well. Also, after 29. Rg7 Red8 30. Nf4 Be8 31 Rfg1 Rd7 32 Rg8+ Kh7 33. Ne6 would seal the win with the direct mating threat of Nf8#, which can only be averted at the cost of heavy material loss. Nice attack by Petrosian.