Theory
The main line continues with the moves 4. cxb5 a6 5. bxa6 Bxa6 followed by Black fianchettoing the f8-bishop. (Black players leery of the double-fianchetto system, where White plays g3 and b3, and fianchettos both bishops, have preferred 5...g6 intending 6.b3 Bg7 7.Bb2 Nxa6! The point is that it is awkward for White to meet the threat of ...Nb4, hitting d5 and a2, when Nc3 may often be met by ...Nfxd5 because of the latent pin down the long diagonal.) Black's compensation for the pawn takes several forms. Firstly, White (who is already behind in development) must solve the problem of developing the f1-bishop. After 6. Nc3 d6, if White plays 7.e4, then Black will play 7...Bxf1, and after recapturing with the king, White will have to spend time castling artificially with g3 and Kg2, as in the line 7....Bxf1 8.Kxf1 g6 9.g3 Bg7 10.Kg2. If White avoids this by fianchettoing the bishop, it will be in a rather passive position, being blocked by White's own pawn on d5.
Apart from this, Black also obtains fast development and good control of the a1–h8 diagonal and can exert pressure down the half-open a- and b-files. These are benefits which can last well into the endgame and so, unusually for a gambit, Black does not generally mind if queens are exchanged; indeed, such an exchange can often usefully remove the sting from a kingside attack by White.
Although the main line of the Benko is considered acceptable for White, there are various alternatives which avoid some of the problems entailed in the main line. The simplest is to just decline the gambit with 4.Nf3 or 4.a4. Another idea, which is popular at the grandmaster level as of 2004, is to accept the pawn but then immediately give it back with 4.cxb5 a6 5.b6. Another popular alternative is 5.e3.
Read more about this topic: Benko Gambit
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