What was the point of Shirov's 47...Bh3? One of the most famous moves ever is Shirov's 47...Bh3!! in Topalov - Shirov, 1998.

[FEN "8/8/4kpp1/3p1b2/p6P/2B5/6P1/6K1 b - - 0 47"]
[Event "Linares"]
[Date "1998"]
[White "Veselin Topalov"]
[Black "Alexey Shirov"]


I've always taken that for granted and never really thought about it. Can anybody give an explanation of why it is the best move?
I interpret the question as follows


  How can a player arrive at ...Bh3?


This is tightly connected to the point behind ...Bh3. With a material advantage, black is clearly looking for a win. Yet, it is well known that opposite colored bishops give the defending side a strong chance to draw. Thus, precise play is required.

Black's plan is


Get the king to e4 by Ke6-f5-e4
Do d5-d4 to limit the enemy bishop
Push a4-a3-a2
Queen one of the pawns


White's plan is


Get the king to d4 by Kg1-f2-e3-d4
Prevent the enemy king from reaching e4
Prevent the enemy from pushing d5-d4
Prevent the enemy king from supporting the black a-pawn


Next, black has to answer the question


  What is the most direct way to make the plan work?


The key point is that black's bishop doesn't really help much in promoting the pawns. It is the king that plays the key role. Thus, sacrificing the bishop to win a tempo makes total sense! In addition, after gxh3, white can no longer create a passed pawn on the kingside! Thus, the bishop sacrifice wins a tempo and ruins white's counter-play. For example

[FEN "8/8/4kpp1/3p1b2/p6P/2B5/6P1/6K1 b - - 0 47"]

1...Bh3! 2.gxh3 Kf5 3.Kf2 Ke4 4.Bxf6 d4 5.Be7 Kd3 6.Bc5 Kc4! 7.Bd6 Kc3 8.Ke2 Kc2 9.Bb4 d3+ 10.Ke3 a3 0-1