Stockfish 8 says 23... Bd6 was an inaccuracy and that 23... Bc7 was better. Why?
The most obvious reason to my mind would be the fact that from c7 you can re-route the bishop via b6 to the a7-g1 diagonal, which is quite important.
Yeah, it's basically an issue of options.
On d6, your bishop really only has access to one interesting diagonal, the b8-h2 diagonal.
It can go to e7, but there aren't really any prospects there.
On c7 it has access to the a7-g1 diagonal, which will likely become very interesting, since it can join in the pressure on f2, or if white moves the f-pawn, create some pressure against the king on that diagonal.
Further, black will very likely want to put a rook on the d-file at some point, and would like to have it unobstructed.
These are human rationalizations of it, though. :)
Honestly, the main reason SF flags it as an inaccuracy is that it hasn't calculated deep enough yet.
Black is just up a piece, and neither move throws that away. Bd6 just makes black take a few more moves to prove a similar advantage as Bc7.
For example, around iteration 34, SF evaluates the two bishop retreats as essentially equal (+4.21 and +4.13 for Bc7 and Bd6 respectively).
EDIT: Fixed some imprecise language I occasionally lapse into :)
On d6, your bishop really only has access to one interesting diagonal, the b8-h2 diagonal.
It can go to e7, but there aren't really any prospects there.
On c7 it has access to the a7-g1 diagonal, which will likely become very interesting, since it can join in the pressure on f2, or if white moves the f-pawn, create some pressure against the king on that diagonal.
Further, black will very likely want to put a rook on the d-file at some point, and would like to have it unobstructed.
These are human rationalizations of it, though. :)
Honestly, the main reason SF flags it as an inaccuracy is that it hasn't calculated deep enough yet.
Black is just up a piece, and neither move throws that away. Bd6 just makes black take a few more moves to prove a similar advantage as Bc7.
For example, around iteration 34, SF evaluates the two bishop retreats as essentially equal (+4.21 and +4.13 for Bc7 and Bd6 respectively).
EDIT: Fixed some imprecise language I occasionally lapse into :)
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