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:
Here, of course, 4.Kf3 allows Nd4+ winning the c-pawn.
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
.. Nxg5 3. c7
.. hxg5 3. h6
The point of the bishop sac is to get the King out of the passed pawns "square"