Find the decisive blow (difficulty 1155)

87654321 abcdefgh

White to move

Show solution
87654321 abcdefgh
  1. b1–b7 key move
  2. b8–b7
  3. a1–b1

What makes it

A rook move and a 4-point investment: mate in four. The forced line runs 7 plies. At 1155, no gift.