Can you describe the power and purpose of Udi as explained in the provided text?

The text from Chapter 35 highlights Udi, or sacred ash, as a powerful form of Prasad and blessing from Sai Baba. Its purpose is twofold. Firstly, it serves as a tangible form of permission to leave Shirdi, as no one receives it for departure unless Sai's heart is pleased. Secondly, it possesses immense healing and protective power. It is described as having a "natural tendency" to remove trouble. This is exemplified by the story of a man whose chronic sleep issues from bad dreams were instantly cured by applying the Udi with faith, demonstrating its ability to resolve problems where all other efforts have failed.