How was Sai Baba's Udi instructed to be used for the boy with the incurable bone-cyst?

As narrated in Chapter 34, after the parents of the suffering boy traveled to Shirdi and pleaded their case, Sai Baba provided a simple yet profound instruction. The parents, with choked voices and pitiful faces, explained their son's condition and their own exhaustion. Sai Baba, described as the embodiment of compassion, reassured them and told them to be free of worry. He then gave them his Udi and specifically instructed them to smear it on the wound. This act of applying the sacred ash directly to the afflicted area was presented as the definitive solution to a problem that expert surgeons and numerous medicines could not solve. The instruction was coupled with Baba's powerful assurance of a positive outcome.