1 Bb5+ Kd8

If 1… Rxb5+? 2 Kxb5 Kd8 3 Kc6 Ke8 and the following is purely technical, inaccurate and without interest (ex : 4 Kd6 Kf8 5 Be3! Kg7 6 e7 etc.)

2 e7+ Kc7 3 e8=R!!

Threats 4 Rxb8, and also 4 Re7+ etc.

3… Kxe8

If 3… Ra8+ 4 Rxa8! This justifies the promotion in Rook (the shortest line following is : 4… Kb7 5 Ra7+ Kb8 (Kd8) 6 Kb6 (Bd6) Kc8 (Kd8) 7 Ra8#): 3 e8=Q? Ra8+! 4 Kb4 Rxe8! Or 4 Qxa8?? stalemate.

4 Bxe8 Kd8 5 Bb5! Kc7 6 Ka6 Kd8/Kb8

If 6… Kc8 7 Bb6 Kb8 8 Bd7 Ka8 9 Bg1 (or Bf2/e3/d4/c5) Kb8 10 Bh2+ Ka8 11 Bc6#

7 Kb6 Kc8 8 Be7 Kb8 9 Ba6 Ka8 10 Bb7+ Kb8 11 Bd6#

 

 

-    Under-Promotion in Rook

-    Double switchback of white Bishop to b5 and a6