This study is very tough. In order to see the key idea, one has to find a lot of difficult preceeding moves...
The first point – after 1...Nxb6 2.cxb6 b1Q 3.b7 Qh1+ 4.Kg7 Qg1+ 5.Kh8 Black has only perpetual. In case of 1...Ke4 2.Na4 a draw is much easier.
2.Re8+ Kf5 3.Re1! Nc1.
Now White can't stop the pawn, so he has to show his own trumps.
The only chance is to build a drawing mechanism with his rook and knight. How to do it?