1. Nb4+? (1. Nc7+ Kb6 2. Rxd6+ +-) 1... Ka5
2. Rxd6 winning the Rook is bad because it's stalemate.
So 2. Nd5 was played and the game carried on.
Now we can see why the first move was so important to get right.
If 1. Nc7+ Ka5 2.Rxd6 b4+
3.Kb2 (3.Kb3 is stalemate) 3... b3 4. Ka3 b2 5. Ra6#
1... Re6+!= 2. Kc5 (2. Kxe6 is stalemate)
2... Re1= (2... Rf6? 3. Kb6 Rf7 4. Ra8+ Kd7 5. Ne5+ or 2... Rd6? kb6 +-)
Unfortunately Black played 1... Rd8+ instead of 1... Re6+!.