2 added diagram
source | link

It's because 1. exf8D+exf8=Q+ Kxh7 2. Rxd7?? would be stalemate. I think it is a even a theoretical draw after 1. exf8D+exf8=Q+ Kxh7 2. Qg7+. Therefore 1. exf8B+exf8=B+! (with check!) is better, since white then can keep an extra piece and win easily.

[FEN "5r2/3qPbkB/8/7P/8/8/8/1K1R4 w - - 0 1"]

1. exf8=Q+ (1. exf8=B+!) Kxh7 2. Rxd7? (2. Qg7+)

It's because 1. exf8D+ Kxh7 2. Rxd7?? would be stalemate. I think it is a even a theoretical draw after 1. exf8D+ Kxh7 2. Qg7+. Therefore 1. exf8B+! (with check!) is better, since white then can keep an extra piece and win easily.

It's because 1. exf8=Q+ Kxh7 2. Rxd7?? would be stalemate. I think it is a even a theoretical draw after 1. exf8=Q+ Kxh7 2. Qg7+. Therefore 1. exf8=B+! (with check!) is better, since white then can keep an extra piece and win easily.

[FEN "5r2/3qPbkB/8/7P/8/8/8/1K1R4 w - - 0 1"]

1. exf8=Q+ (1. exf8=B+!) Kxh7 2. Rxd7? (2. Qg7+)
1
source | link

It's because 1. exf8D+ Kxh7 2. Rxd7?? would be stalemate. I think it is a even a theoretical draw after 1. exf8D+ Kxh7 2. Qg7+. Therefore 1. exf8B+! (with check!) is better, since white then can keep an extra piece and win easily.