White plays Ng4 as first move. If Black plays Kf1 or Kg1 or Kh1 then Rd8 followed by Rd1 mates irrespective of Blacks second move. The subtle solutions are when Black plays Kf3 or Kh3 as first move. In the first of these cases White plays a clever waiting move, say Kd6. Black is forced to play g2, then Kd3 completes the mate. If Black plays Kh3 as first move then White plays Nh2. Now if Black plays Kh3 or g2 then Qh7 for White mates, while if Black plays f3 then White mates with Rh8.