user27496: rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
[Event "?"]
[Site "?"]
[Date "????.??.??"]
[Round "?"]
[White "?"]
[Black "?"]
[Result "*"]

1. e4 e5 2. Nf3 Nf6 3. Nxe5 d6 4. Nf3 Nxe4 5. Nc3 Nxc3 6. dxc3 Be7 7. Be3 O-O 8.
Qd2 Be6 9. h4 Nc6 10. Be2 Ne5 11. Ng5 h6 12. Nxe6 fxe6 *

I (black) like playing the Petroff and my opponent answered with a Nimzowitsch attack. I tought that my position would be quite decent. After 11. .. h6 the engine turned from +-0 to +2 without losing any material. I understand that the move was not perfect positionally but could somebody explain the huge change?

user-1: The main reason that this move is weak is that you already have all major white figures targeting fields surrounding black king, and you are adding problems to it because the only option you have is to take the knight weakening the position even further by making g6 undefended. Notice that even if the white bishop would capture the pawn at h6, he would not be in as bad position as you could imagine, taking into accounts all the threats that you would have to control.
But what is the mistake you ask? The mistake is that you did not move the attacked bishop first. The best is to move it to f5 and then you are almost there. You could move to d7 but it seems a more defensive move than it should be. Notice that d7 both attacks and defends. And now after white castles 0-0-0, you can safely play h6.
Simply put, you forgot a supporting move. You have other moves besides h6 but h6 is not that bad after 0-0-0.
You have to play safe, that is all. h6 is simply to quick and obvious choice. But, it is so weak choice at that moment that it must be considered a blunder. You are not in that bad position after all.

user23278: If we see from black prospective its pieces don't have a clear plan.For me as a player from black side I will try to bring the other rook on the king side. So as to defend the position strongly because black knight and bishop are having very less squares on the other hand white will bring his other rook on the king side with castling.Pushing those king side pawns makes it look horrible for because even if white is able to create one open file on the king side like g and h file.I don't see any defense for black and after then white can even sacrifice his bishop to get his queen in the game.If you think of taking black king to the other side of the board to me it looks to be very difficult.

user28279: It sounds like you want a more concrete answer than "your light squares are now very weak", "white has the bishop pair", and "you've just created a hook for white to utilize by pushing the g-pawn to g5 [and you likely won't have h5 as a response]". Obviously your engine can give you the most concrete justifications, but I would argue that after 13.0-0-0, white will play f4 driving your knight back (f4 right away can be met by Bxh4+, so no need to rush it), followed by g4 and g5, busting open your kingside without even losing a pawn, I believe. The rook from d1 will join in. If you, as black, ever take hxg5, the h-file will open for white's h1 rook, and if not white will play f5 and then take on Bxh6, thereby having the bishop and queen also join in the attack.

user70: White has easy ideas like 1. Nxe6 fxe6 2. Bxh6 gxh6 3. Qxh6 with ideas of Rh3 to g3.

user20688: To add to the existing answers, you just gave up your light squared bishop for no apparent reason, while creating many light square weaknesses around your king with the same move! Now white is the only player with a light squared bishop and you will have a tough defensive job if white plays correctly.

user307: By playing 11...h6, you created a weakness on g6, and you created a target on h6, now it is easier for white to open up a position around your king by pushing their g-pawn. White also has a potential bishop sacrifice on h6.
In general it is best to avoid pawn moves in front of your king, especially when facing a pawn storm.

27496: <pre><code>rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
[Event &quot;?&quot;]
[Site &quot;?&quot;]
[Date &quot;????.??.??&quot;]
[Round &quot;?&quot;]
[White &quot;?&quot;]
[Black &quot;?&quot;]
[Result &quot;*&quot;]

1. e4 e5 2. Nf3 Nf6 3. Nxe5 d6 4. Nf3 Nxe4 5. Nc3 Nxc3 6. dxc3 Be7 7. Be3 O-O 8.
Qd2 Be6 9. h4 Nc6 10. Be2 Ne5 11. Ng5 h6 12. Nxe6 fxe6 *
</code></pre>
<p>I (black) like playing the Petroff and my opponent answered with a Nimzowitsch attack. I tought that my position would be quite decent. After 11. .. h6 the engine turned from +-0 to +2 without losing any material. I understand that the move was not perfect positionally but could somebody explain the huge change?</p>
 -1: <p>The main reason that this move is weak is that you already have all major white figures targeting fields surrounding black king, and you are adding problems to it because the <em>only option</em> you have is to take the knight weakening the position even further by making g6 undefended. Notice that even if the white bishop would capture the pawn at h6, he would not be in as bad position as you could imagine, taking into accounts all the threats that you would have to control.</p>
<p>But what is the mistake you ask? The mistake is that you did not move the attacked bishop first. The best is to move it to f5 and then you are almost there. You could move to d7 but it seems a more defensive move than it should be. Notice that d7 both attacks and defends. And now after white castles 0-0-0, you can safely play h6.
Simply put, you forgot a supporting move. You have other moves besides h6 but h6 is not that bad after 0-0-0.</p>
<p>You have to play safe, that is all. h6 is simply to quick and obvious choice. But, it is so weak choice at that moment that it must be considered a blunder. You are not in that bad position after all.</p>
 23278: <p>If we see from black prospective its pieces don't have a clear plan.For me as a player from black side I will try to bring the other rook on the king side. So as to defend the position strongly because black knight and bishop are having very less squares on the other hand white will bring his other rook on the king side with castling.Pushing those king side pawns makes it look horrible for because even if white is able to create one open file on the king side like g and h file.I don't see any defense for black and after then white can even sacrifice his bishop to get his queen in the game.If you think of taking black king to the other side of the board to me it looks to be very difficult.</p>
 28279: <p>It sounds like you want a more concrete answer than &quot;your light squares are now very weak&quot;, &quot;white has the bishop pair&quot;, and &quot;you've just created a hook for white to utilize by pushing the g-pawn to g5 [and you likely won't have h5 as a response]&quot;. Obviously your engine can give you the most concrete justifications, but I would argue that after 13.0-0-0, white will play f4 driving your knight back (f4 right away can be met by Bxh4+, so no need to rush it), followed by g4 and g5, busting open your kingside without even losing a pawn, I believe. The rook from d1 will join in. If you, as black, ever take hxg5, the h-file will open for white's h1 rook, and if not white will play f5 and then take on Bxh6, thereby having the bishop and queen also join in the attack.</p>
 70: <p>White has easy ideas like <code>1. Nxe6 fxe6 2. Bxh6 gxh6 3. Qxh6</code> with ideas of <code>Rh3</code> to <code>g3</code>.</p>
 20688: <p>To add to the existing answers, you just gave up your light squared bishop for no apparent reason, while creating many light square weaknesses around your king with the same move! Now white is the only player with a light squared bishop and you will have a tough defensive job if white plays correctly.</p>
 307: <p>By playing <code>11...h6</code>, you created a weakness on <code>g6</code>, and you created a target on <code>h6</code>, now it is easier for white to open up a position around your king by pushing their g-pawn. White also has a potential bishop sacrifice on h6.</p>
<p>In general it is best to avoid pawn moves in front of your king, especially when facing a pawn storm.</p>
