Can you explain the story of the wooden plank that Maharaj used for sleeping?

Certainly. The story of the plank is detailed in the Introduction. Seeing that the Masjid was dilapidated, a devotee named Nanasaheb Dengle brought a thick wooden plank for Maharaj to sleep on, rather than the bare ground. However, Maharaj used it in a very unusual way. As described in Chapter 0, instead of placing it on the floor, he hung the plank from the rafters of the Masjid using old rags. Miraculously, these flimsy rags supported the weight of both the plank and Maharaj's body. He would also place four lit lamps at the corners while sleeping on it.