Find the decisive blow (difficulty 1035)

12345678 hgfedcba

Black to move

Show solution
12345678 hgfedcba
  1. a3–b4 key move
  2. g3–c3
  3. e5–e1

What makes it

The bishop sets up a forced mate in four. The forced line runs 7 plies. At 1035, a fair workout.