Find the decisive blow (difficulty 1275)

87654321 abcdefgh

White to move

Show solution
87654321 abcdefgh
  1. h3–g5 key move
  2. g7–g6
  3. g5–f7

What makes it

A knight move and 2 points of material given up: mate in six. No check, no capture: just a quiet move, and the defence caves in. 11 half-moves of forced play.