Lesson
White to move. Main line: 1. Rxf7+ Kg8 2. Rg7+ Kh8 3. Rxd7+ Kg8 4. Rg7+
1.Rxf7+ moves the rook off g7 and uncovers the bishop’s diagonal from c3 to h8. The check therefore comes from the bishop, allowing the rook to capture on f7 without losing a tempo.
After ...Kg8, the rook returns with Rg7+, checking directly and forcing ...Kh8. The cycle restarts, and Rxd7+ removes the queen while once again uncovering the bishop. Each swing has one checking half and one capturing half.
A windmill requires two coordinated pieces: one gives the discovered check when the other moves away, and the moving piece can return with a direct check. Before starting the cycle, count how many targets the mobile piece can collect and where the king can deviate.