Find the decisive blow (difficulty 1380)

12345678 hgfedcba

Black to move

Show solution
12345678 hgfedcba
  1. c4–d5 key move
  2. e3–e4
  3. b7–f7

What makes it

The bishop strikes, a 2-point investment, and the attack wins. No check, no capture: just a quiet move, and the defence caves in. It calculates 16 plies deep.