user16057: Why sacrificing a pawn in this situation, 19... h5 is slightly more preferable by engine over more materialistic g6 or Kh8? This is in fact what Ding chose to play actually. Obviously, this is to protect from 20. Bh6, but I myself try not to give away pawns when I have reasonable alternatives.
Why protecting dark squares was so important for Blacks even in the presence of the dark-squared bishop?
[FEN ""]
[Event "6th Norway Chess 2018"]
[Site "Stavanger NOR"]
[Date "2018.05.28"]
[Round "1.3"]
[White "Nakamura, Hikaru"]
[Black "Ding, Liren"]
[Result "1/2-1/2"]
[ECO "A18"]
[WhiteElo "2769"]
[BlackElo "2791"]
[PlyCount "73"]
[StartPly "38"]
[EventDate "2018.05.28"]

1.c4 Nf6 2.Nc3 e6 3.e4 d5 4.cxd5 exd5 5.e5 Ne4 6.Nf3 Bf5 7.d3 Nxc3 8.bxc3 c5 9.d4 Qa5 10.Bd2 Nc6 11.Be2 Be7 12.O-O O-O 13.a4 Rac8 14.Ra2 a6 15.Rb2 c4 16.Ne1 b5 17.axb5 axb5 18.Bg4 Bxg4 19.Qxg4 h5 20.Qxh5 b4 21.Qg4 Rfe8 22.Nf3 Qa3 23.Rc2 b3 24.Rcc1 Qa2 25.Qf5 Ra8 26.Bg5 Qe2 27.Qd7 Bxg5 28.Nxg5 Ne7 29.e6 f6 30.Nf7 Ng6 31.Nh6+ gxh6 32.Qf7+ Kh8 33.Qxf6+ Kh7 34.Qf7+ Kh8 35.Qf6+ Kh7 36.Qf7+ Kh8 37.Qf6+


user14202: Blacks counter play is based on ...b4  If he plays a slow move White  can thwart b4 with Nc2 (which also stops Blacks Queen from entering on the a-file) and then attack on the kingside.  Some ideas are f2-f4-f5-f6, Rf1-f3-h3.  After Nc2 White can also take his time and just play Rf-b1 and Black is passive.

user12873: It comes down to what's worth more in the moment. By playing ...h5 Black gained one tempo (since White had to waste a tempo moving the queen back to g4). The question is: in that position is one tempo worth more than the h-pawn?
The answer is yes, because both sides are in a race. White is trying to checkmate Black's king before Black can make serious progress on the queenside. By giving up the lone h-pawn (which Black wasn't going to use in the near future anyway), Black gains one more tempo in this race. Later on when the game heats up and becomes sharper, it's clear that every tempo is important. 
In normal positions it's not worth giving up a pawn for a tempo, but the position in the game was a "do or die" case. Either your attack is faster, or you lose on the spot.
Also, Black's dark-squared bishop is able to defend, but only for so long. So gaining a tempo still helps quite a bit.

16057: <p>Why sacrificing a pawn in this situation, <code>19... h5</code> is slightly more preferable by engine over more materialistic <code>g6</code> or <code>Kh8</code>? This is in fact what Ding chose to play actually. Obviously, this is to protect from <code>20. Bh6</code>, but I myself try not to give away pawns when I have reasonable alternatives.</p>

<p>Why protecting dark squares was so important for Blacks even in the presence of the dark-squared bishop?</p>

<pre><code>[FEN ""]
[Event "6th Norway Chess 2018"]
[Site "Stavanger NOR"]
[Date "2018.05.28"]
[Round "1.3"]
[White "Nakamura, Hikaru"]
[Black "Ding, Liren"]
[Result "1/2-1/2"]
[ECO "A18"]
[WhiteElo "2769"]
[BlackElo "2791"]
[PlyCount "73"]
[StartPly "38"]
[EventDate "2018.05.28"]

1.c4 Nf6 2.Nc3 e6 3.e4 d5 4.cxd5 exd5 5.e5 Ne4 6.Nf3 Bf5 7.d3 Nxc3 8.bxc3 c5 9.d4 Qa5 10.Bd2 Nc6 11.Be2 Be7 12.O-O O-O 13.a4 Rac8 14.Ra2 a6 15.Rb2 c4 16.Ne1 b5 17.axb5 axb5 18.Bg4 Bxg4 19.Qxg4 h5 20.Qxh5 b4 21.Qg4 Rfe8 22.Nf3 Qa3 23.Rc2 b3 24.Rcc1 Qa2 25.Qf5 Ra8 26.Bg5 Qe2 27.Qd7 Bxg5 28.Nxg5 Ne7 29.e6 f6 30.Nf7 Ng6 31.Nh6+ gxh6 32.Qf7+ Kh8 33.Qxf6+ Kh7 34.Qf7+ Kh8 35.Qf6+ Kh7 36.Qf7+ Kh8 37.Qf6+
</code></pre>
 14202: <p>Blacks counter play is based on ...b4  If he plays a slow move White  can thwart b4 with Nc2 (which also stops Blacks Queen from entering on the a-file) and then attack on the kingside.  Some ideas are f2-f4-f5-f6, Rf1-f3-h3.  After Nc2 White can also take his time and just play Rf-b1 and Black is passive.</p>
 12873: <p>It comes down to what's worth more in the moment. By playing ...h5 Black gained one tempo (since White had to waste a tempo moving the queen back to g4). The question is: in that position is one tempo worth more than the h-pawn?</p>

<p>The answer is yes, because both sides are in a race. White is trying to checkmate Black's king before Black can make serious progress on the queenside. By giving up the lone h-pawn (which Black wasn't going to use in the near future anyway), Black gains one more tempo in this race. Later on when the game heats up and becomes sharper, it's clear that every tempo is important. </p>

<p>In normal positions it's not worth giving up a pawn for a tempo, but the position in the game was a "do or die" case. Either your attack is faster, or you lose on the spot.</p>

<p>Also, Black's dark-squared bishop is able to defend, but only for so long. So gaining a tempo still helps quite a bit.</p>
