Find the decisive blow (difficulty 1485)

12345678 hgfedcba

Black to move

Show solution
12345678 hgfedcba
  1. f3–e2 key move
  2. f1–e2
  3. g6–g4

What makes it

A bishop move and a 3-point investment: mate in eight. A piece slips off the line and the attack opens up behind it. It calculates 15 plies deep.