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The puzzle https://lichess.org/training/kFzbF has position

[Title "Black to move"]
[FEN "r3rnk1/1bq2pp1/1ppb1n1p/p2p4/3P1B2/P2B1N1P/1PPQNPP1/3RR1K1 b - - 1 16"]

and the solution is Rxe2 Qxe2 Bxf4.

Lichess's Stockfish gives this line is -5, and gives all other moves 0. Why?

I ran through several lines and don't see any unifying themes; for example, here is a line where I traded a bunch of pieces (while picking near-top moves):

[Title "Black to move"]
[FEN "r3rnk1/1bq2pp1/1ppb1n1p/p2p4/3P1B2/P2B1N1P/1PPQNPP1/3RR1K1 b - - 1 16"]

1... Rxe2 2. Qxe2 Bxf4 3. c4 Re8 4. Qc2 Rxe1 5. Rxe1 c5 6. dxc5 bxc5 7. cxd5 Bxd5 8. Nh4 Ne6 9. Bc4 Bxc4 10. Qxc4 Be5 11. Qe2 Bd4 12. Nf5 Qb7 13. Nxd4 cxd4 14. Rb1 Qb3 15. Qd1 Qa2 16. b4 axb4 17. axb4 Ne4 18. Qf1 Nf4 19. Re1 Qd5 20. f3 Ng3 21. Qf2 d3 22. Qxg3 Ne2+ 23. Rxe2 dxe2 24. Qe1 Qd4+ 25. Kh2 Qd1 26. Qc3 Qb1

and it's not clear to me that black is better until near the end.

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  • Well, trading pieces, including the bishop pair, is the silliest thing to do with Black (when it could be wonderfully used to pester the white rooks or to attack the white King). Even though chess wisdom tells you to trade officers when having the advantage...Insofar this exact line is rather meaningless. Sep 24, 2022 at 18:55

2 Answers 2

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There is a lot going on, but first of all notice that the rook worth 5 is less than the bishop and knight worth 6. That said, that alone is nothing like enough to give a -5 advantage for black.

When you look at the exact transformation you get a few more clues: before the transformation, white's best placed piece is probably the dark square bishop, pointed down towards the queen and not easily attacked by black pawns without significantly weakening the black king. Eliminating it makes it that much harder for white to attack black's very well defended king and removes a defender of the white king.

That raises the question of why not to capture the bishop directly. The reason is that by taking the knight first, not only do you eliminate one of the defenders of the potentially weak d4 pawn, you also give yourself the queen-bishop battery, which combined with the potential for a pawn break c6 to c5 opening up the light-squared bishop (played later) puts a ton of pressure down towards the white king. In the continuation you posted, white is later forced into a losing two knights vs rook end game.

Basically, you end one point up with a nasty bishop pair pointed down towards white's relatively defenceless king (as compared with black's king for instance). That said I kind of feel like a rating of 1624 is pretty low to catch the nuances of the position - especially when you consider that in the game this is pulled from, a 2242 rated player totally missed the idea.

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  • -3, -5 and -100 are basically the same evaluation. They just mean Black is winning. Trading a rook for two minor pieces is often a decisive advantage.
    – David
    Oct 25, 2022 at 17:18
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1...Rxe2 is the best move simply because a bishop and knight are better than a rook. Evaluations given by recent versions of Stockfish can sometimes be a bit impractical for humans, since if Stockfish can recognize a position as a type that it would always/almost always win, then it will evaluate it accordingly.

For humans, a more practical evaluation of your position might be something like -2.50, instead of -5. Dividing a big evaluation Stockfish gives by 2 is a good rule of thumb that I've found works decently for many positions, if you want an evaluation that is more realistic for human play.

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