user-1: In The wisest things ever said about chess by GM Soltis, he quotes this about tempi and gives the following example:

Tempi should not be counted but weighed.

[FEN "r2q1rk1/pp1b1nb1/n2p1pp1/3Pp2p/2N1P2B/2N2P2/PP2B1PP/R2Q1RK1 w - - 0 1"]
[Site "Istanbul"]
[Date "2000"]
[White "Gelfand"]
[Black "Shirov"]

  1. a4             Qb8           
  2. Nb5 (2...Bxb5 3. axb5 Nc7)           Rc8           
  3. Bf2            Bh6           
  4. Qb3            Nc5           
  5. Qa3             *

The point of the example is that white started with 2 extra tempi in a King's Indian structure, that allowed him to place pressure on the queenside.
My question is, why didn't black play 2...Bxb5 3. axb5 Nc7 to gain chances to win one of the b-pawns?

user8258: After 4.Qb3 in your proposed line, there aren't any real chances to win the b5 pawn.  If Black continues to add pressure with ...Qe8, then after the knight on c4 moves, the bishop on e2 will provide enough protection to defend the b5 pawn.
In the meantime, white gets the advantages of the bishop pair, the semi-open a-file for his rook, and a black knight that has no future on c7.  Black doesn't appear to my eyes to have quick enough counterplay on the kingside while white can continue building up forces on the queenside and has a potential pawn break with the move b5-b6.  The resulting position in your proposed variation would be static and strategically miserable for black.

user4418: 
why didn't black play 2...Bxb5 3. axb5 Nc7 to gain chances to win one
  of the b-pawns?

1) The bishop on d7 is a particulerly fine bishop fighting for control of some very important white squares contested by white pawns. Getting rid of it seems a shame.
2) It's not clear that black is going to win the resulting white pawn on b5 any time soon and in the meantime it is very annoying indeed.
3) Maybe white is going to follow up with b6 and either that pawn is going to be a monster or if it is swapped off white is going to end up with a knight on b6 instead supported by the bishop which is going to move to f2. The white squared bishop is going to come to b5 and there is no corresponding bishop for black.
4) It is going to be difficult for black to compete for the a and c files.
All in all only a masochist is going to go down that route.

-1: <p>In <em>The wisest things ever said about chess</em> by GM Soltis, he quotes this about tempi and gives the following example:</p>

<blockquote>
  <p>Tempi should not be counted but weighed.</p>
</blockquote>

<pre><code>[FEN "r2q1rk1/pp1b1nb1/n2p1pp1/3Pp2p/2N1P2B/2N2P2/PP2B1PP/R2Q1RK1 w - - 0 1"]
[Site "Istanbul"]
[Date "2000"]
[White "Gelfand"]
[Black "Shirov"]

  1. a4             Qb8           
  2. Nb5 (2...Bxb5 3. axb5 Nc7)           Rc8           
  3. Bf2            Bh6           
  4. Qb3            Nc5           
  5. Qa3             *
</code></pre>

<p>The point of the example is that white started with 2 extra tempi in a King's Indian structure, that allowed him to place pressure on the queenside.</p>

<p>My question is, <strong>why didn't black play 2...Bxb5 3. axb5 Nc7 to gain chances to win one of the b-pawns</strong>?</p>
 8258: <p>After 4.Qb3 in your proposed line, there aren't any real chances to win the b5 pawn.  If Black continues to add pressure with ...Qe8, then after the knight on c4 moves, the bishop on e2 will provide enough protection to defend the b5 pawn.</p>

<p>In the meantime, white gets the advantages of the bishop pair, the semi-open a-file for his rook, and a black knight that has no future on c7.  Black doesn't appear to my eyes to have quick enough counterplay on the kingside while white can continue building up forces on the queenside and has a potential pawn break with the move b5-b6.  The resulting position in your proposed variation would be static and strategically miserable for black.</p>
 4418: <blockquote>
  <p>why didn't black play 2...Bxb5 3. axb5 Nc7 to gain chances to win one
  of the b-pawns?</p>
</blockquote>

<p>1) The bishop on d7 is a particulerly fine bishop fighting for control of some very important white squares contested by white pawns. Getting rid of it seems a shame.</p>

<p>2) It's not clear that black is going to win the resulting white pawn on b5 any time soon and in the meantime it is very annoying indeed.</p>

<p>3) Maybe white is going to follow up with b6 and either that pawn is going to be a monster or if it is swapped off white is going to end up with a knight on b6 instead supported by the bishop which is going to move to f2. The white squared bishop is going to come to b5 and there is no corresponding bishop for black.</p>

<p>4) It is going to be difficult for black to compete for the a and c files.</p>

<p>All in all only a masochist is going to go down that route.</p>
