Find the decisive blow (difficulty 1035)

87654321 abcdefgh

White to move

Show solution
87654321 abcdefgh
  1. c7–a6 key move
  2. b8–a8
  3. a3–b5

What makes it

A knight move forces mate in four. It is a discovered attack: the moving piece clears the way for the one behind it. The forced line runs 7 plies.