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I was looking at the computer analysis for a game I played and I need some help to understand a line found by the computer.

After a balanced game (started with a Torre Attack) we arrived at the following position: mid game starting position

You see at the bottom the line suggested by the computer instead of my 23..h6 (that didn't lead anywhere in the end).

This interesting line starts with 23..f3 24. Nxf3. I understand why eventually we arrive at the position below with 27..Be6. But the computer suggests 28.a4 instead of moving the rook. Can you please explain the engine analysis here?

enter image description here

Why doesn't White, for example, exchange their rook with Black’s rook and then play 29. Qe2?

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After:

23 ..f3 24 Nxf3 Bg4 

You are threatening Bxf3 to win a piece, since the g2 pawn is pinned. So sacrificing the f3 pawn gives you great initiative and attacking options, and keeps white pinned down.

Even if white moves the king to avoid the pin, with say Kf1, you can take in f3 with the rook and if white captures back with the pawn Bh3 is mate:

[FEN "2br1rk1/p5pp/1p1p2q1/1PnRp1N1/2P1Pp1P/P7/2B1QPP1/4R1K1 b - - 0 1"]

1. f3 Nf3 2. Bg4 Kf1 3. Rf3 gxf3 4. Bh3#

Running the king to the other way, h1, is also equally bad, because Rf4 becomes undefendable:

[FEN "2br1rk1/p5pp/1p1p2q1/1PnRp1N1/2P1Pp1P/P7/2B1QPP1/4R1K1 b - - 0 1"]

1. f3 Nf3 2. Bg4 Kh1 3. Bxf3 gxf3 4. Rf4 Qf1 5. Rxh4 Qh3 6. Rxh3#

So as you can see, the attack becomes quite strong, so the engine suggests white to sacrifice the knight to gain counter attacking options, and not lose on the spot. Even though the position is still lost regardless.

I understand why eventually we arrive at the position below with 27..Be6. But the computer suggests 28.a4 instead of moving the rook

Why doesn't White, for example, exchange their rook with Black’s rook and then play 29. Qe2?

Well the exchange sacrifice on d5 isn't particularly bad considering the position, and how the d file becomes closed, but you get even further down material. Also other than Rxd8 you only have Rd6 to not sacrifice the exchange but that loses instantly to Rxd6 followed by Qxf2+ leading to mate.

The answer to "Why not Rxd8?" is: The engine didn't have enough time to think. The suggested move by Stockfish 10 in this position is in fact Rxd8 followed by either Qe3 or Qe2, as you can see in the following picture:

enter image description here

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  • Thank you. How did you insert the two chessboards in the message? – GabiM Oct 24 '19 at 11:45
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    @GabiM No problem, You insert them with the code button in the editor and typing in something like [FEN "2br1rk1/p5pp/1p1p2q1/1PnRp1N1/2P1Pp1P/P7/2B1QPP1/4R1K1 b - - 0 1"] Most chess editors, like lichess analysis board, provide you with the fen string so you only need to copy it here. If you click on edit in my answer you can see the code i used for my live boards. – Isac Oct 24 '19 at 11:57
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In the first position, f3 is good because it will open the white K after Nxf3 Bg4 then Bxf3 or maybe Rf3 in some cases as the light squares will be very weak (the analysis gave up the Nf3 with Ne5 immediately since the position was so bad that giving up a piece was "best"...if that is best, then it is dead lost). What is left is a HUGE Nc5 compared to a pathetic Bc2.

In the second diagram, you still have that Nc5 vs. the Bc2, but white is already down a piece. That is just lost, and a matter of time.

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    is right. White is down a piece, so exchanging a set of Rooks on top of that will only help Black. "When ahead in material, exchange pieces; when behind, exchange pawns." If 28. Rxd8 Rxd8 29. Qe2 how do you answer Rd4 with threats against both e4 and c4? – Arlen Oct 22 '19 at 20:56
  • @Arlen, you are right. To be honest, I did not even calculate from the point of Qe2. I already just assessed the position (down a piece, and great Nc5 vs horrible Bc2 being the major points, but also the weakened white K to some degree). That said, after Rd4, the most natural move, white cannot even defend c4 at all.Bc4 is next. – PhishMaster Oct 22 '19 at 21:03
  • So basically the e4 and c4 pawns are lost anyway so the computer suggest 28.a4 so that after 28..Bxd5 29. cxd5 the white will at least have a stronger pawn structure than the alternative given by me with 29.Qe2. Thanks – GabiM Oct 23 '19 at 8:40
  • @GabiM I did not seriously consider anything but moving, and thus trading, the Rd5. To play a4 leaves white down a rook, which is more than enough to win easily. It is elementary from there really. – PhishMaster Oct 23 '19 at 9:59

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