The amazing faraway subpromotion seems at first paradoxical, as the new piece, the knight, does not have the time to take part in the mate ! However, it's absolutely necessary, as 1. bxa8=Q or 1. bxa8=B leave black stalemated, and 1. bxa8=R allows black 1. ..., Kxg2, removing the pin on the bishop, so black can go on with 2. ..., Bxa7, and a mate in 3 would be impossible
The amazing faraway subpromotion seems at first paradoxical, as the new piece, the knight, does not have the time to take part in the mate ! However, it's absolutely necessary, as 1. bxa8=Q or 1. bxa8=B leave black stalemated, and 1. bxa8=R allows black 1. ..., Kxg2, removing the pin on the bishop, so black can go on with 2. ..., Bxa7, and a mate in 3 would be impossible