What specific incident mentioned in Chapter 40 sparked the author's desire to write about Sai Baba's life?

According to Chapter 40, a key inspiration for writing the Shri Sai Satcharitra was the sight of Sai Baba grinding grain. The author describes the wonder he felt watching the saint set up the grinding stone, spread His legs, and pour the grain to be ground. This act filled the author's mind with a strong desire to describe all of Sai's deeds, believing that by doing so, impurities and sins would be destroyed. The text explicitly states that this image of Sai grinding grain is 'the root of this book.'