Abdel Irada: <<•>How not to play the Dragon<•>>By springing this trap in his pet variation of the Yugoslav Attack, Fischer gave his opponent a "free" opening lesson that cost the latter at least the exchange. (This, of course, is why Black has to exchange on d4 *before* playing ...b5 in this line.)
<<•> 12. Nxc6, Qxc6 >
Black has to recapture. If he tries to interpolate with a counterattack, his situation only becomes worse: (a) 12. ...b4? 13. Nxb4
or (b) 12. ...a5?! 13. a4 and White wins essentially as in the text.
<<•> 13. Nd5 ... >
Now Black does have choices, six of them, in fact. Unfortunately, they all lose at least the exchange.
Note that White threatens 14. Nxe7, winning the queen, so anything Black does must prevent this.
<<•> (1) 13. ...Qb714. Nxf6†, Bxf6
15. Bd5, Qb8
16. Bxa8, Qxa8
17. Bd4
>
White has won the exchange, and with his last move blunts Black's counterplay. From here, White's attack with h4-h5 will go ahead as planned and at leisure, since Black has no adequate threats of his own.
Other defenses are no better.
< (2) 13. ...Nxd5?
14. Bxd5
>
Here White wins an entire rook.
< (3) 13. ...Qd7
14. Nb6, Qb7
15. Nxa8
>
Here, too, White wins the exchange, and will continue with 16. Bh6 and the stock kingside pawn storm.
< (4) 13. ...Qe8
14. Nc7, Qc6
15. Nxa8
>
This transposes to line (3) and the outcome will be the same.
< (5) 13. ...Re8?
14. Nxf6†, Bxf6
15. Bd5
>
As in line (2), White picks up a full rook.
< (6) 13. ...Kh8?
14. Nxf6, Bxf6
15. Bd5
>
Again White comes out a rook to the good.
And so opening theory is verified through the bitter tuition of practical punishment for error. Black cannot play ...b5 without first interpolating an exchange of knights on d4. (And even this contains a positional pitfall, since White can exchange bishop for knight on f6 and ruin Black's pawn structure: 11. ...Nxd4 12. Bxd4, b5?! 13. Bxf6, Bxf6 14. Nd5
. To make matters worse, Black's d-pawn will fall, leaving him a pawn down and in a worse position.)