user-1: 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?

user18168: The Nbd2 --> Nc4 plan to protect the bishop doesn't work because c4 is attacked by black's pawn. Stockfish takes a more direct route to kick the pin:
[fen "r1b1kb1r/pp3ppp/1q2p3/n2pPn2/1P1P4/P4N2/1B3PPP/RN1QKB1R w KQkq - 0 1"]

1. Nc3 {White can kick the pin in a moment.} (1. Nbd2 {This move also poorly develops the knight - and as you can see, c4 is covered anyway.}) 1...Bd7 2. Na4

And the pin is already gone. Na5 didn't really do anything in the first place. It was interesting while it lasted, but that wasn't long. It wasn't exactly a bad move, even (lichess would call it an "inaccuracy," I'd wager); however, there are better ideas here, such as the simple developing move Bd7.

user12873: The plan you outlined with ...Bd7, ...Rc8, ...Nc4 is a good strategical plan, but there are a few concrete reasons it doesn't work well:

In the opening you should be focusing on developing all your pieces and castling. Your plan with maneuvering the knight to c4 will move the same piece three times.
At depth 33, Stockfish gives 10.Nc3 Bd7 11.Rb1!, breaking the pin on the b2-bishop and attacking the a5-knight. Now you have to move the knight before your a8-rook manages to reach c8. If you go ahead with 11...Nc4 then 12.Bxc4 dxc4 is poor for you. Your new pawn on c4 is mainly a weakness at the moment, and White may blast open the centre with d5 to attack your uncasted king.
Also, White could always play 10.Qa4+. If you block with 10...Bd7 then 11.Qxa5 wins the knight. So you have to respond with retreating the knight: 10...Nc6.


-1: <p>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. </p>

<pre><code>[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
</code></pre>

<p>However, I don't find this move at the database of chesstempo I use to use for my correspondence games.</p>

<p><a href="https://i.stack.imgur.com/KgH7C.png" rel="noreferrer"><img src="https://i.stack.imgur.com/KgH7C.png" alt="chesstempo database before Na5"></a></p>

<p>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).</p>

<p>What's my misunderstanding here?</p>
 18168: <p>The Nbd2 --> Nc4 plan to protect the bishop doesn't work because c4 is attacked by black's pawn. Stockfish takes a more direct route to kick the pin:</p>

<pre><code>[fen "r1b1kb1r/pp3ppp/1q2p3/n2pPn2/1P1P4/P4N2/1B3PPP/RN1QKB1R w KQkq - 0 1"]

1. Nc3 {White can kick the pin in a moment.} (1. Nbd2 {This move also poorly develops the knight - and as you can see, c4 is covered anyway.}) 1...Bd7 2. Na4
</code></pre>

<p>And the pin is already gone. Na5 didn't really do anything in the first place. It was interesting while it lasted, but that wasn't long. It wasn't exactly a <em>bad</em> move, even (lichess would call it an "inaccuracy," I'd wager); however, there are better ideas here, such as the simple developing move Bd7.</p>
 12873: <p>The plan you outlined with ...Bd7, ...Rc8, ...Nc4 is a good strategical plan, but there are a few concrete reasons it doesn't work well:</p>

<ul>
<li>In the opening you should be focusing on developing all your pieces and castling. Your plan with maneuvering the knight to c4 will move the same piece three times.</li>
<li>At depth 33, Stockfish gives 10.Nc3 Bd7 11.Rb1!, breaking the pin on the b2-bishop and attacking the a5-knight. Now you have to move the knight before your a8-rook manages to reach c8. If you go ahead with 11...Nc4 then 12.Bxc4 dxc4 is poor for you. Your new pawn on c4 is mainly a weakness at the moment, and White may blast open the centre with d5 to attack your uncasted king.</li>
<li>Also, White could always play 10.Qa4+. If you block with 10...Bd7 then 11.Qxa5 wins the knight. So you have to respond with retreating the knight: 10...Nc6.</li>
</ul>
