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?
In order to build this mechanism, White has to put his rook under two attacks. Black has an extra queen and the right to move, but it is not enough for a win.
5...Kf6 (5...Qxe4 6.Nd6+; 5...Kxe4 6.Nd2+) 6.Nd6.
Then White gives checks from d6 and e8. 1/2-1/2