Find the decisive blow (difficulty 1035)

12345678 hgfedcba

Black to move

Show solution
12345678 hgfedcba
  1. d4–c3 key move
  2. b3–d2
  3. c3–d2

What makes it

A bishop move forces mate in four. The blow comes from a discovered attack — one piece moves, another fires. 7 half-moves of forced play.