In the Sicilian Najdorf poisoned pawn variation what is the difference between including the moves 7...h6 8 Bh4 (as many top players do) and not including them?
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1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6 6. Bg5 e6 7. f4 Qb6 (7...h6 8. Bh4 Qb6 9. Qd2 Qxb2) 8. Qd2 Qxb2
One reason is that it avoids a certain line that you would get after playing 7...Qb6, which is 8.Nb3
If Black plays 7...h6 first, 8.Bh4 Qb6 9.Nb3 is no longer possible because Black can play 9...Qe3+ winning the f4-pawn. This would not have been the case in the 7...Qb6 line since the bishop would have still been on g5, protecting the pawn.
Another reason is that sometimes Black would like to play ...g5.
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1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6 6. Bg5 e6 7. f4 Qb6 (7...h6 8. Bh4 Qb6 9. Nb3 Qe3+ 10.Qe2 (10.Be2 Nxe4) 10...Qxf4) 8. Nb3 Be7 (8...Qe3+ 9.Qe2)
According to the Game Database of ChessTempo, 7....Qb6 is actually more popular than 7....h6. At the very top level, both moves have their supporters, e.g. Anand and Grischuk for 7....Qb6 and Vachier-Lagrave and Nepomniachtchi for 7....h6.
Both moves can easily transpose, as the main line of the Poisoned Pawn goes 7....Qb6 8.Qd2 Qxb2 9.Rb1 Qa3 10.e5 h6 11.Bh4.
After 7....Qb6, white has the option to play 8.Nb3, as mentioned by Seth-Riley. With the inclusion of 7....h6 8.Bh4, 9.Nb3 is not possible because of 9....Qe3+.
However, after 7....h6 8.Bh4 Qb6, white has the option to play 9.a3. If black takes the pawn with 9....Qxb2, white can trap the queen by 10.Na4. Instead black should continue his development, for instance with 9....Be7. Now white can play 10.Bf2, which is of course possible thanks to the inclusion of 7....h6 8.Bh4.
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1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6 6.Bg5 e6 7.f4 h6 (7...Qb6 8.Qd2 (8.Nb3) Qxb2 9.Rb1 Qa3 10.e5 h6 11.Bh4) 8.Bh4 Qb6 9.a3 (9.Nb3 Qe3+)(9.Qd2 Qxb2 10.Rb1 Qa3 11.e5) Be7 (9...Qxb2 10.Na4) 10.Bf2
Originally the Q that you have asked would cover a book with both Variations pros & cons with Games from Top level being annotated . There are no two very different significant ideas stemming from the above lines .
You can play both and find out yourself which one you are comfortable in .
I would like to bring out one difference which is when you play the h6 and then Bh4 then Qb6 I would say that you play Be7 also which removes the Knight pin . As I can say this variation is quite difficult for White to make the K-side attack which is generally the main line .
1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6 6.Bg5 e6 7.f4 Qb6 8.Qd2 Qxb2 9.Rb1 Qa3 10.e5 dxe5 11.fxe5 Nfd7 12.Bc4 Bb4 13.Rb3 Qa5 14.O-O O-O 15.Bf6
The above is one of the main lines in Najdorf Poison Pawn and extreme ultra sharp . If Black makes one single mistake he will face the danger of getting exploded . So compared to the above h6 is quite a good line to consider .
You can even play h6 later and not only on 7th move as such no such a great difference can be pulled out .
