Find the decisive blow (difficulty 1215)

87654321 abcdefgh

White to move

Show solution
87654321 abcdefgh
  1. e2–b5 key move
  2. e7–a3
  3. b2–a1

What makes it

Best defence still loses — mate in six. A quiet move, soft as anything, and yet there is no reply. The forced line runs 11 plies.