White to move. How should white proceed?
a) 1...Kb1, 2. Rb2+ Ka1, 3. Nc3 Rh2 (3...Rc1, 4. Ra2#), 4. Rb1#
b) 1...Rh3+, 2. Nc3 (threatening 3. Ra2# or Rc1#) Rxc3+, 3. Rxc3 Kb1, 4. Rc4 Ka1, 5. Rc1#
You will want to play Nc3 at some point, but you don't want to block the rook's access to the 1st/2nd rank by playing it first- to do so allows black to play the rook up the board, even to h7 where it is immune to capture due to the stalemate. So, the most direct line here would be 1.Rc2 which threatens 2.Nc3 followed by Ra2#. There is no defense now- 1.....Kb1 is met by 2.Rb2+ followed by 3.Nc3 and Ra2#.