I find Na5 a good resource for black in this line of the french, profiting from the fact that white's b-pawn is binned to the bishop. As far as I can understand this position with my limited chess, I find it a strong move, winning space on queenside with the plan of trying Bd7-Rc8-Nc4 on c-file.
[FEN "r1b1kbnr/pp3ppp/1qn1p3/2ppP3/3P4/2P2N2/PP3PPP/RNBQKB1R w KQkq - 3 6"] 6. a3 Nh6 7. b4 cxd4 8. cxd4 Nf5 9. Bb2 Na5
However, I don't find this move at the database of chesstempo I use to use for my correspondence games.
Stockfish neither considers it. It even don't play 10....Nbd2 (it plays Nc3) to defend on c4 (which I would expect), and doesn't wonder at all about Nc4 (it simply trades his bishop for the knight; apparently it should result an advantage on the center).
What's my misunderstanding here?