6

In the following position, why is g4 so powerful (+4.1 according to stockfish), when g4 is normally considered a weakening move?

[Title "Why is g4 so powerful here?"]
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[Startply "28"]

1.e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. d3 d6 6. O-O Be7 7. Be3 b5 8. Bb3 O-O 9. a3 Bg4 10. Nc3 Nd4 11. Bxd4 exd4 12. Nd5 Nxd5 13. Bxd5 Rb8 14. h3 Bh5
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  • 3
    I'm not using engine so bare with me if I miss. The +4.1 may due to various factors. After bishop drops to g6 you can play Nxd4 threatening nc6. Now you are already a pawn up material wise. Also now it allows you to further expand on the kingside with f4 potentially threatening to trap the g6 bishop. Also in case when black defends the C6 fork white can still go nc6 and chop the e7 bishop. Now white is left with a strong lsb while blacks lsb is trapped on g6/h7
    – cmgchess
    Sep 17, 2023 at 6:54
  • You're winning a pawn!
    – David
    Sep 17, 2023 at 20:01

3 Answers 3

4

The first thing to note is that 14...Bh5 is a blunder. Much better would have been 14...Bd7 keeping an eye on the weak light squares on the queenside and particularly c6.

The move 15. g4 is very strong here because it comes with tempo and frees the knight to take the d4 pawn, again with tempo. It is only one pawn but white's pieces are better and white has the initiative. White is calling the shots and black has to respond rather than make their own plans.

Here is some analysis:

[Title ""]
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[Startply "28"]

1.e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. d3 d6 6. O-O Be7 7. Be3 b5 8. Bb3 O-O 9. a3 Bg4 10. Nc3 Nd4 11. Bxd4 exd4 12. Nd5 Nxd5 13. Bxd5 Rb8 14. h3 Bh5 (14...Bd7 15. Nxd4 Bf6 16. c3 Bxd4 17. cxd4 c6 18. Ba2 Qf6 {and black is regaining the pawn}) 15. g4 Bg6 16. Nxd4 {threatening the knight fork on c6} Rb6 17. Nc6 {anyway} Qd7 18. Nxe7+ Qxe7 19. a4 {quick, before black has time to play c6} bxa4 20. Rxa4 Qf6 21. b3 c6 22. Bc4
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Normally moves like h3-g4 are considered weakening because they open up your king, and also provide pawn levers for the opponent. For example after h3-g4, if opponent plays h5, then they can conceivably open lines against your king after ...hxg4.

But in this position:

  1. Black is in no shape to attack White's king. If anything it is White who will attack Black's king (they have far more space on the kingside).
  2. Dynamic considerations - after ...Bg6, White gets to eat a free pawn on d4 (which simultaneously threatens f4 to roll Black off the board on the kingside).

Point 2 is the more important factor - you can see how the engine evaluation jumped after ...Bh5, since it loses a pawn.

0

In this case, Black's bishop will be trapped on g6 by White's pawns. f7 is pinned by White's bishop so the only retreat square is h7. That white bishop is rather difficult to kick out so it will take at least three moves for Black to re-manuever his bishop.

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