White win with ♘a8:
1.-♘a8 , … c8= ♘
2.-♘b6 .. ?
And White wins with the move ♖b7 ++ inevitably.

A variant would be:
1.-♘ a8, …♖b4
2.-♘b6 .. ?
And White wins with the move ♖b7++ inevitably.

Another variant would be:
1.- ♘a8, …♖g8
2.-♘b6 .. ?
And White wins with the move ♖b7++ inevitably.