Find the decisive blow (difficulty 855)

87654321 abcdefgh

White to move

Show solution
87654321 abcdefgh
  1. a7–b6 key move
  2. a8–b8
  3. a6–a8

What makes it

A bishop move forces mate in two. A piece slips off the line and the attack opens up behind it. At 855, gentle enough to spot.