Why should Black capture with different pawns in these similar Spanish game variations?

Question

As a general “rule”, I have read that pawn captures toward the center should be preferred. I have found 2 different positions which look similar but the best way for Black to recapture with the pawn is different.

[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR"]
[Title "Spanish Variation 1"]
[StartFlipped "1"]

1. e4 e5 2. Nf3 Nc6 3. Nc3 Nf6 4. Bb5 Bc5 5. Bxc6 bxc6!?

In the variation above, Black captures toward the center but Stockfish labels it as an inaccuracy and suggests capturing with the d-pawn instead. The advantage goes from 0 to around +0.6.

[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR"]
[Title "Spanish Variation 2"]
[StartFlipped "1"]

1. e4 e5 2. Nf3 Nc6 3. Bb5 Nf6 4. Qe2 Bc5 5. Bxc6 dxc6!?

In the second variation, Black does not capture toward the center this time. But Stockfish still labels is as an inaccuracy and suggests capturing with the b-pawn instead. The advantage goes from 0 to around +0.8.

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The positions look very similar to me. Why does Stockfish recommend recapturing in different ways in these positions?

Answer 1

As you attempt to understand chess, one thing that you are probably coming to realize is that almost every rule has exceptions. This is one of the most interesting ones.

The "general rule" to capture towards the center, is strategic, and aims at using the pawns to control the center of the board. The exceptions mostly have to do with tactics. Here we have a situation that is common in the Ruy Lopez. Can White win the e5 Pawn by removing the c6 Knight?

In your first position. 5. .bxc6 can be answered strongly with Nxe5 and recovering the Pawn will be difficult. The Knight can supported with d4, which will come with gain of tempo against Bc5. On the other hand, after ..dxc6 6.Nxe5, Black easily recovers the pawn with 6...Bxf2+ 7.Kxf2 Qd4+. The loss of a central Pawn is fully compensated by a fast and easy development.

In the second position, 5..bxc6 is playable because if 6.Nxe5 Qe7 recovers the Pawn and the supporting move d4 is not available. White has given up the two Bishops to no purpose.