Every chessplayer who likes to fianchetto his king's bishop in the opening quickly learns the value of this bishop. It is the main (and in some cases the only) defender of his king. As my first coach used to say, pointing at my Bg7 in the Dragon variation of the Sicilian Defense: "As long as this guy is alive, you are not going to get checkmated!" In some cases you might want to sacrifice some material rather then allow the trade of your fianchettoed bishop. (We discussed this subject here.)
Now let's look at the next position:
We have a textbook example of a 'good knight' versus a 'bad bishop', so the knight shouldn't be traded for the bishop, right? From the other side, Bf6 is a fianchettoed bishop which is supposed to protect his king, and therefore shouldn't be traded. Does it mean that the trade 18.Nxf6 is bad for both players?
After the move 18.Nxf6+ was played, GM Eduard Gufeld (who was a big fan of the King's Indian bishop) came to Kasparov and said:
"Who?" asked Kasparov.
"Both of you!" answered Gufeld.
In his notes to the game, Kasparov says that at that point he found Gufeld's joke amusing, but at the end of the game he was wondering if Gufeld was right indeed and all Black's problems started with the trade of his fianchettoed bishop.
Nevertheless, there is a common and well-known situation where a trade of the fianchettoed bishop for a knight is fully justified. Robert James Fischer was one of the first who made this unusual trade typical:
Yes, after the trade Black's kingside gets weaker and in the game Fischer had to withstand a very dangerous attack. But look at those ugly, isolated c3 and c4 pawns on the open c-file; they were completely doomed!
One of the greatest positional players of all times, Tigran Petrosian, also liked the idea:
And then another world champion embraced the unusual trade:
I have to confess that when I studied this idea I was very skeptical at first. You cannot erase the years of the Sicilian Dragon experience that easily! Yet, at the end I was converted:
This memorable game brought me the final GM norm (and the title!) as well as allowed our team to win silver Olympic medals!
This interesting but double-edged idea should be in the tool kit of every chessplayer!