How did Sai Baba manage to sleep on that tiny plank? Does the book explain how it could hold his weight?

Chapter 10 addresses this very question, explaining that Baba's ability to sleep on the narrow plank was a display of his divine powers, or Siddhis. The text states that the plank, tied with mere rags, was supported by the "Great Siddhis residing within Him" and was thus "merely for name’s sake." It goes on to explain that Baba possessed powers like 'Anima,' the ability to become as minute as a tiny particle. This power allowed his movements to be as subtle and easy as those of flies, insects, or ants, which is why no one ever perceived him getting on or off the plank. His ability to do this was not a matter of physical mechanics but of his supreme spiritual attainment.