White has just played 17.Rd1-e1, attacking Black's e-Pawn. While it can be defended by Re8, White can triple the major pieces on the e-file to attack it three times. While it can still be defended three times, White will be able to shift the attack to the Queenside or the Kingside, as circumstances dictate.
Black can't play e7-e6 because the e6 square is not covered adequately and because the d-Pawn is not protected after d5xe6. Black's only counterplay is to advance the a-Pawn, eventually attacking the Bishop on b3, which will nevertheless find a safe square on c4 after a preparatory move or two. Then it is difficult to see how Black can create a real attack on the a- and b-files. If pieces are exchanged then Black's advanced Queenside Pawn(s) will become weak and subject to encirclement.
How did Korchnoi continue? It's a mystery, because the game isn't on any of our databases. It's hard to see how any player, even with the defensive skills of a world class player like Korchnoi, could survive Black's position.
