I usually play the French, but have seen some Najdorf videos, so I will try my best until someone better comes along
You could try these videos.
https://www.youtube.com/watch?v=pemkjtICTLg
https://www.youtube.com/watch?v=JuLYgClRe-Q
https://www.youtube.com/watch?v=QuL191Z3vAo
https://www.youtube.com/watch?v=BPg1bSeJYNg
Hope this helped
Does such a natural move as a6 (especially with 2 white knights aiming at b5) require any additional justification ?!? 
Does such a natural move as a6 (especially with 2 white knights aiming at b5) require any additional justification ?!?
That's a great question! The potential downside of a6 or h6 in any game is: do you really need to protect against Bg5 (or Bb5) so much so that you miss one move of developing your pieces? I'm still trying to develop a feel for when -- in general -- such a move is "too defensive" vs "appropriately defensive".
Does such a natural move as a6 (especially with 2 white knights aiming at b5) require any additional justification ?!?
That's a great question! The potential downside of a6 or h6 in any game is: do you really need to protect against Bg5 (or Bb5) so much so that you miss one move of developing your pieces? I'm still trying to develop a feel for when -- in general -- such a move is "too defensive" vs "appropriately defensive".
The main reason for this move is to allow e5 the next move. For example:
Now white gets a good out post
I read the following comment in two quite separate places.
The best example of this very different approach to the game is the move 5...a6 in the Sicilian Najdorf 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6. The move is an indirect defense of the thematic square d5.
Defense of d5? Is this saying that a6 followed by b5 then prevents white from playing Bc4 which would attack the d5 square?