solskytz: (to THE pawn)
not exactly comprehension...
in Wade's place I would go 17...Ne7 rather than ...f5.
Bobby was thinking about the endgame very early on. After the Q exchange, his only advantage is the advanced position of his central pawns.
But how to exploit it without any pieces out?
Well - if he can exchange his e pawn for the opponent's d pawn (no matter that he helps his adversary's development in so doing), he creates an asymmetrical pawn formation: 4:3 in his favor on the Q-side, 2:3 against him on the K-side.
In itself, this could still be pretty balanced. A 4:3 majority is kind of clumsy to exploit. It takes quite time to mobilize, and the enemy also has his chances on the other wing (and he does try, oh yes does he try...)
BUT - if he can weaken and paralyze black's Q-side minority (such as after 18. Bxc6 bc), then he has a long-term, stable, clearly crushing advantage.
Then he can sit and wait. Let Black exhaust himself on the K-side. We can even accept some concessions (such as the interesting B-manoeuvre - g3, h2, g1). We can suffer this, even with opposite colour B's).
Sooner or later black has no means to further his position on the K-side.
this could be quite frustrating in playing against Fischer: his wasn't a very preventive or prophylactic style. In many games he would create a long-term advantage and then let you attack for 10-15 moves. You think that you control the flow of the game, but he already saw that nothing will come out of your efforts, and when you have nothing left to do, then he kills you, not softly.
As the black initiative (in the face of sure Q-side loss, by reason of the pawn formation) wears off, White starts to advance in the Q-side. c4 can't be prevented. If the B has to move from d5, f3 will hang and the white R will have access to the Q-side, where pawns can no longer defend one another (a key point in every variation, and the reason I wouldn't let Fischer play 18.Bxc6).
Question: What can black do?
Answer: play into white's hands, simplify, allow an exchange of opposite colour B's (a drawing factor) and a couple of pawns, so that the situation is made even clearer: Black has nothing to show for his efforts on the K-side; strangely enough, it is exactly there that White's winning advantage is now manifest.