user8291: In the Two Knights Defense, after 
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]
1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.Ng5 d5 5.exd5 Na5 6.Bb5+ c6 7.dxc6 bxc6

8. Bd3 has become fashionable recently, and is considered a serious way for White to play for a win. 
Yet, is seems an artificial move, going against basic opening principles, in particular blocking the d-Pawn. What are the main points of this line, especially in comparison to the "old" 8. Be2 move, and are there any typical traps or pitfalls to be aware of? 

user32178: I think it's helpful to think of bishop hoping to go to f1, after O-O and Re1. Good illustrative game is Lev Aronian v. Vidit Gujrathi Kolkata 2019 (Rapid & Blitz): https://www.chessgames.com/perl/chessgame?gid=1981131

user18809: I think this move illustrates the problem with "opening principles". Sometimes they contradict one another!
For example, 8.Be2 may appear more natural because it does not block the "d" pawn. However, it also places the bishop in a passive square as, as indicated by Mike Jones, does not allow for Ne4 later on.
Finally, Qf3-related ideas may still be playable after 8.Bd3

user12873: The point is that after 8...h6, White can now play 9.Ne4. Comparatively, after 8.Be2 h6, White must go for 9.Nf3 e4, when the knight is hit again.
You're right that 8.Bd3 blocks the d2-pawn from advancing, but this isn't such a huge price to pay. The c1-bishop can get into the game via b3 followed by Bb2.

user9684: I think the main reason behind Bd3 is to provide a "safe" retreat for the knight and keeps to e-file open.  Both Nf3 and Nh3 have liabilities. A retreat to e4 also allow a further lose of time from f5, but this may be offset by black's LSB having less mobility.  The other option is Bf1, which while playable, also goes against basic opening principles.
Although this may apply here, paraphrasing Alekhine, to punish unconventional moves (Na5) you may have to play unconventional moves.

8291: <p>In the Two Knights Defense, after </p>

<pre><code>[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]
1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.Ng5 d5 5.exd5 Na5 6.Bb5+ c6 7.dxc6 bxc6
</code></pre>

<p><code>8. Bd3</code> has become fashionable recently, and is considered a serious way for White to play for a win. </p>

<p>Yet, is seems an artificial move, going against basic opening principles, in particular blocking the d-Pawn. What are the main points of this line, especially in comparison to the "old" <code>8. Be2</code> move, and are there any typical traps or pitfalls to be aware of? </p>
 32178: <p>I think it's helpful to think of bishop hoping to go to f1, after O-O and Re1. Good illustrative game is Lev Aronian v. Vidit Gujrathi Kolkata 2019 (Rapid &amp; Blitz): <a href="https://www.chessgames.com/perl/chessgame?gid=1981131" rel="nofollow noreferrer">https://www.chessgames.com/perl/chessgame?gid=1981131</a></p>
 18809: <p>I think this move illustrates the problem with &quot;opening principles&quot;. Sometimes they contradict one another!</p>
<p>For example, 8.Be2 may appear more natural because it does not block the &quot;d&quot; pawn. However, it also places the bishop in a passive square as, as indicated by Mike Jones, does not allow for <code>Ne4</code> later on.</p>
<p>Finally, <code>Qf3</code>-related ideas may still be playable after <code>8.Bd3</code></p>
 12873: <p>The point is that after 8...h6, White can now play 9.Ne4. Comparatively, after 8.Be2 h6, White must go for 9.Nf3 e4, when the knight is hit again.</p>

<p>You're right that 8.Bd3 blocks the d2-pawn from advancing, but this isn't such a huge price to pay. The c1-bishop can get into the game via b3 followed by Bb2.</p>
 9684: <p>I think the main reason behind Bd3 is to provide a "safe" retreat for the knight and keeps to e-file open.  Both Nf3 and Nh3 have liabilities. A retreat to e4 also allow a further lose of time from f5, but this may be offset by black's LSB having less mobility.  The other option is Bf1, which while playable, also goes against basic opening principles.</p>

<p>Although this may apply here, paraphrasing Alekhine, to punish unconventional moves (Na5) you may have to play unconventional moves.</p>
