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.