Ideas and tactics behind the 4.h4 (or 5.h4) attack vs. the Gruenfeld? What are the main ideas and tactical themes behind the early h2-h4 attack vs. the Gruenfeld? I mean, the 1.d4 Nf6 2.c4 g6 3.Nc3 d5 4.h4 (or 4.Nf3 Bg7 5.h4, which seems to me a fairly related line) variations?
First, I think that a lot of it is simply to get away from the heavier theory of other lines. Stockfish does not think highly of this move, and it does not make sense that such a move would be best from a purely theoretical standpoint. From a practical standpoint, that is another ball of wax since some of the positions get very messy. After all, who does not get a little queasy when the opponent starts throwing the h-pawn at you?

White's idea is pretty straight forward: It is obviously to prepare a direct attack against the black king, and there are actually lines that black voluntarily castles into to it. I do not believe that theory has come up with a definitive evaluation just yet. Can black play a line that will render h4 semi-useless, or not? I am skeptical that this is good for any advantage, or worse.

Despite the aggressive intentions, the lines below show that black has a lot of counter-punching available in the position.

Here is a couple of lines with my comments on the early 4.h4.

 [FEN ""]

 1. d4 Nf6 2. c4 g6 3. Nc3 d5 4. h4 c5 {This has to be best just striking back in the center will full force. This seems to render h4 pointless for now since there is clearly not going to be any king of direct attack with the center so open.} (4... dxc4 {This would be the most principled move, meeting a flank attack with action in the center, but it is not well-prepared, and white gets too much center.} 5. e4 c5 6. d5) (4... c6 {This is a line, but it seems passive, and seems to fall right into white's plans as far as I am concerned.}) 5. dxc5 d4 {Leading to a very complex game.} 6. Nb5 e5 $1 7. b4 Nc6 8. Bg5 Nxb4 9. Bxf6 Qxf6 10. Nc7+ Kd8 11. Nxa8 Bf5 12. Rc1 Bxc5 {And black is probably better.}


Here are some lines and comments on 5.h4.

 [FEN ""]

  1. d4 Nf6 2. c4 g6 3. Nc3 d5 4. Nf3 Bg7 5. h4 {Unlike the last line, c5 does not work so well here so white seems justified in throwing in Nf3 also before 5.h4.} dxc4 $1 (5... c5 $2 6. dxc5 {And because there is no d4, black will struggle to regain the pawn.}) 6. e4 {And black has several ideas here.} c5 $1 {Striking back at the center is the main line.} (6... O-O {In this line, black says, "I am ahead in development, so I do not see how your attack will work.} 7. h5 Nxh5 8. Bxc4 Bg4 9. Be3 e5 (9... Nc6 $5) 10. dxe5 Nc6 {Was Morozevich-Popov Moscow blitz 2013.}) 7. d5 {And there are three moves from here, 0-0, Qa5, and b5!? The last two are very complex.} b5 $5 {For example.} 8. Nxb5 $2 Qa5+ 9. Nc3 Nxe4 10. Bd2 (10. Qa4+ $4 Qxa4 11. Nxa4 Bd7 $19) 10... Nxc3 11. Qc1 Nd7 $17 (11... Nxa2 $5)