user31736: Recently I've been looking into the Scheveningen variation of the Sicilian and from what I have read it seems like the Keres attack is a big threat and you need great calculation ability to stop it as Black. This is why numerous books have recommended just playing the Najdorf and then e6 on the 6th move saying that the Najdorf (a6) stops the Keres attack.
My question is how the move a6 stops the Keres attack. If I could have a specific game or something since I can't find a concrete answer anywhere else.

user14079: In the Najdorf variation (1.e4 c5 2.Nf3 d6 3.d4 cd4 4.Nxd4 Nf6 5.Nc3 a6), the c8-h3 diagonal remains open and Black has two hits (Nf6, Bc8) on the square g4. If White plays 6.g4?? in the spirit of the Keres attack, Black already gets a decisive advantage with 6...Bxg4, grabing a free pawn with tempo.
In contrast, the Keres variation 6.g4!? is possible against the Scheveningen because 5...e6 (or 2...e6 is Black chose another move order) blocked the Bc8 diagonal and reduced their control of g4: then 6...Nxg4?? 7.Qxg4 e5 8.Nf5 g6 9.Bc4 would be winning for White.
Edit Feb 2022 :
In spite of the loss of a pawn, some players are toying with the idea of playing 6.g4? against the Najdorf anyway. World Blitz Champion Maxime Vachier-Lagrave used it in a demo bullet game against GM Bilel Bellahcene, live on French TV. Bellahcene quips: "hey, you're using my own variation!"

31736: <p>Recently I've been looking into the Scheveningen variation of the Sicilian and from what I have read it seems like the Keres attack is a big threat and you need great calculation ability to stop it as Black. This is why numerous books have recommended just playing the Najdorf and then e6 on the 6th move saying that the Najdorf (a6) stops the Keres attack.</p>
<p>My question is how the move a6 stops the Keres attack. If I could have a specific game or something since I can't find a concrete answer anywhere else.</p>
 14079: <p>In the Najdorf variation (1.e4 c5 2.Nf3 d6 3.d4 cd4 4.Nxd4 Nf6 5.Nc3 a6), the c8-h3 diagonal remains open and Black has two hits (Nf6, Bc8) on the square g4. If White plays 6.g4?? in the spirit of the Keres attack, Black already gets a decisive advantage with 6...Bxg4, grabing a free pawn with tempo.</p>
<p>In contrast, the Keres variation 6.g4!? is possible against the Scheveningen because 5...e6 (or 2...e6 is Black chose another move order) blocked the Bc8 diagonal and reduced their control of g4: then 6...Nxg4?? 7.Qxg4 e5 8.Nf5 g6 9.Bc4 would be winning for White.</p>
<p><em>Edit Feb 2022</em> :
In spite of the loss of a pawn, some players are toying with the idea of playing 6.g4? against the Najdorf anyway. World Blitz Champion Maxime Vachier-Lagrave used it in a <a href="https://twitter.com/manutiss/status/1492445838018326530" rel="nofollow noreferrer">demo bullet game</a> against GM Bilel Bellahcene, live on French TV. Bellahcene quips: &quot;hey, you're using my own variation!&quot;</p>
