Find the decisive blow (difficulty 1035)

12345678 hgfedcba

Black to move

Show solution
12345678 hgfedcba
  1. c7–c5 key move
  2. b4–a5
  3. a1–c3

What makes it

A pawn move forces mate in four. 7 half-moves of forced play. Rated 1035 — no gift.