Can you describe the strange plank Sai Baba used to sleep on and how he was able to use it?

Certainly. Chapter 10 details this unique sleeping arrangement. Baba slept on a narrow wooden plank, described as being just four cubits long and one span (about nine inches) wide. This plank was suspended from the rafters of the mosque, hung at both ends by old rags, like a swing. The chapter notes that his movements were imperceptible; no one ever witnessed how he got on or off this precarious plank. It clarifies that this feat was not due to the plank's strength but was possible because of Baba's Great Siddhis (powers), making the plank's physical support merely for show.