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Though queenside castling is less common, it is important enough to study. If your opponent has castled queenside, sometimes you have an opportunity to expose their King by sacrificing your Bishop for 2 pawns, as showed below. (I have removed other pieces from the board to avoid distraction.)

[FEN "2kr4/ppp5/8/8/8/4B3/8/4K3 w - - 0 1"]

1.Bxa7 b6 2.Bxb6 cxb6 

What factors should one consider before deciding to offer this sacrifice? To put it another way, in what positions is this sacrifice favourable and in what positions would it be unfavourable?

Edit As per comment from xaisoft, I am posting a board with other pieces.

[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]

1.e4 e5 2.d4 d6 3.Bc4 Nf6 4.Nf3 Nc6 5.dxe5 dxe5 6.Nxe5 Nxe5 7.Qe2 Bd7 8.Nc3 Qe7 9.Be3 O-O-O 10.Bxa7 b6 11.Bxb6 cxb6
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Ofter enough, your opponent will defend his pawn rather than trying to trap the bishop, once he has castled. – Nikana Reklawyks Dec 6 '12 at 14:12
I agree, but if they choose to do that, you can simply "abort" the sacrifice and retract your bishop. Here I want to analyse the variation where they choose to play along and accept the sacrifice. – Masked Man Dec 6 '12 at 14:30
@ap. - Would you mind posting a board that has all the pieces? – xaisoft Dec 6 '12 at 15:31
@xaisoft Well, the board was only meant to illustrate the pattern. I did not put the other pieces lest the answers get distracted by that specific position. I want to learn the general principles to be observed in such cases. If you insist, I will put the other pieces too. – Masked Man Dec 6 '12 at 16:04
Am I misreading things? On your second board, after 10...b6?, why 11. Bxb6? Why not 11. Ba6#? I think the danger to white from Bxa7 isn't ...b6, but rather the fact that it costs time. – supercat Mar 20 '14 at 2:44
up vote 3 down vote accepted

I'd argue you never want to play such a bishop sacrifice if your opponent is willing to take it.

Therefore, I think your question is rather “when can I take the undefended ♟ a7”.

To answer this, I'd say you need

  • Either a defense on the bishop (with a rook or a knight)

  • Or a crushing attack if you can sacrifice it. A queen around, or a rook coming on the open c-file, with pieces supporting that are mandatory. If the king can't easily escape to e8, that's better.

  • Nothing better to do. That's just a far pawn, and it takes 2 tempi to take eat if you want to save your bishop. Your opponent need not take bigger of an advantage in the meantime, that you could defend from before taking the pawn.

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In other words, make sure you have a material advantage. – xaisoft Dec 6 '12 at 15:31

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