If 13... Qb6, attacking a pawn, black's knight is in danger of 14. h3. But black wins two pawns in compensation for his knight with Nxf2, 15. Qxf2, Qxb2 Not an overwhelming margin, but the position favors white.
Game submitted for analysis.
Analysis finished:
1. d4 g6 2. e4 Bg7 3. Nf3 d6 4. Bg5 Nf6 5. Bd3 O-O 6. Nc3 c6 7. O-O Re8 8. e5 dxe5 9. dxe5 Ng4 10. Qe2 Qc7 11. Rfe1 Nh6 12. Qd2 Ng4 13. Bf4 f6?? (13... Nd7 14. e6 Nde5 15. exf7+ Kxf7 16. Nxe5+ Bxe5 17. Bxe5 Nxe5 18. Qg5) (13... f6 14. Bc4+ e6 15. exf6 Qd7 16. Qxd7 Bxd7 17. fxg7 Kxg7) (13... f6 14. Bc4+ e6 15. exf6 Qd7 16. Qxd7 Bxd7 17. fxg7 Kxg7)
The computer concluded, as I did, that neither side made any blunders until 13. ...f6
It only so happens that white made small positional grabs.
My analysis:
1. d4 queens pawn opening, followed by g6 with the bishop fianchetto, allows 2. e4, with the dreaded twin center pawn opening for white, grabbing the most space possible and two lines for the bishops, and extra squares for both knights. This opening is technically called the Robatsch Defense transposed, according to my opening book.
Black's compensation, the fianchettoed bishop, is powerful, but in my estimation covers only one color. White's game controls both colors more equally.
3. Nf3 a logical developing move, hitting the center and even countering black's claim to the dark squares. d6 continues challenging the dark squares and opens a line for the Queen bishop, also reasonable.
4. Bg5 is a pet move I came up with, out of the opening book. This move pins the king pawn to the queen, blocks the g6 pawn, and if h6, black's castled pawn structure is somewhat weakened and loses time. White retreats to safety, losing nothing.
Nf6 removes the pin, but blocks the fianchettoed bishop, black's main piece. The knight attacks white's undefended king pawn.
5. Bd3 defends the pawn and develops a piece. The bishop move allows castling kingside. 0-0 castles black.
White's bishops are more active than Black's. White's pawns are more centralized and advanced than Black's. The knights are equal in strength, and Black has castled while white has not committed himself to castling on either side, with no danger on the king yet.
6. Nc3 develops a new piece and attacks the center again, before castling. c6 challenges the center, but occupies a critical square for the queen knight, blocking it in. The pawn does limit the movement of white's queen knight.
7. 0-0 castles whites king on the kingside, preparing to develop the rooks in the volatile center. Re8 challenges white's plans to control the center.
8. e5 forces black to commit himself to a structure. He either can take or move the knight, and neither is palatable. the pawns are exchanged, and the knight has to move.
Black chooses 9...Ng4 which looks aggressive, but is a hasty advance. It has no support and puts the knight on a square where advance isn't recommended and retreat is just as bad.
The computer cannot see this, but look at the squares the knight can move to; none of them look very good. All of them trade or lose the knight, which results in far more limited options for the piece. This move weakens the piece itself in this position, a less tangible weakness, as the position could change. But for now, the piece is less useful and vulnerable. Better is Nd5.
10 Qe2 develops the queen on the dangerous e-file and defends the pawn at e5. Qc7 removes the pin on the e-pawn and attacks white's e-pawn.
The queen has limited scope here. Note how few squares the queen commands. She is not on an active file, she is attacking a well defended pawn, she no longer defends the Rook on e8, is blocked in by her own pieces and pawns on several sides.
Black's structure is limiting his options. Note how the queen rook cannot move, the queen knight cannot move to an active square without blocking the queen, the bishop cannot move to a square that does not block in the queen, the e-pawn, or allows a bishop trade which weakens pawn structure. The knight on g4 is still barely engaged in the center and is hanging vulnerably to pawn attacks. The queen bishop is blocked in by its own lightsquare pawns, the queen isn't as dangerous on d8 or c7 as white's queen, developing the queen to the queenside takes her majesty far away from her royal consort and avoids the center. Due to white's queen, knight, bishops, and pawns, the queenside is also dangerous to Black's queen who would be posted out there by herself, with open lines of attack against her shortly. Black's pawn structure is fairly strong, but cramped. He commands less space than white.
11. Rfe1 doubles rook and queen on the e-file, forming a battering ram. Nh6 is far too passive.
12. Qd2 threatens to win the knight outright, it must retreat. Nf5 would be better, but it returns to g4, trying to be aggressive when it is not effective in that role here.
13. Bf4 defends the pawn again, and threatens a discovery against the queen next turn with a pawn move.
13...f6 loses to the tactical scenarios described above. Before the error, blacks' game was down by slightly more than two points, according to chess.com analysis (*2000 rated).
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