This bishop and pawn ending with the same-colored bishops must have been like child's play for Mr. Korchnoi. Korchnoi would have been a world champion, if Karpov was not in his way several times. But, there are lots of would-have-beens in chess, so let's leave it at that. Some are destined to die on the slope, never reaching the summit.
The problem is, white has pawns on white squares and they can be attacked by black. Black has an offensive move in f5, but he has to be vigilant about the white king's possible foray into his queenside. As always in minor piece and pawn endings, tempo or zugswang come into play. Definitely, black had the initiative here.
Korchnoi moved 53...Kd6 54. Bb5 Be6 ( preparing f5 ) 55. Bf1 f5 56. gf5...or else the H pawn will become passed. Play continued 56...Bf5 and now you begin to see that the white bishop will be torn between the pawns on a4 and h3. So, 57. Bg2 so black can't play his bishop to d7, but 57...Ke5 protects the e pawn and the black bishop is freed from its defensive duties. Narancic played 58. Ke1 ( 58. Kc3 g4 59. hg4 Bg4 60. Kc4 Bd7 and black is winning ) and 58... Bd7 and black wins. If 59. b3, then 59... Be6 or 59...g4. 0-1.