1.G5! So how can black react to this? Not 1...Nxg5. See the pawn on the c-file go claim his reward? 1...hxg then 2.h6 Kf6, 3.Bxe4 and then what to do? ...g5? 4.Bf5! You can not chase two wild pawns on both sides of the field!

Here is the problem with 1.Bxe4:

1.Bxe4? Kxe4
2.g5 Ke5/f5
3.gxh6 Kf6!
4.Kh3

Here, of course, 4.Kf3 allows Nd4+ winning the c-pawn.

4.............Ng5
5.Kg4 Ne6
6.f3 Kf7!

And, now, how does white make progress? 7.Kf5 again allows 7....Nd4+ winning the c-pawn.

Pawn break with 1. Bxe4 Kxe4 2. g5

1. .. Nxg5 3. c7

2. .. hxg5 3. h6

The point of the bishop sac is to get the King out of the passed pawns "square"