According to the provided source, what is the key prerequisite for experiencing the power of Sai's Udi?

The source material from Chapter 35 emphasizes that faith is the crucial prerequisite for the Udi to be effective. When the gentleman from Bandra, who was suffering from bad dreams, sought a remedy, a Sai devotee told him, "If you too keep faith, the Udi will manifest its influence." He was further instructed to "Keep devotion in your heart; then see the influence of this Udi." The text suggests that the Udi's power to immediately remove trouble is its natural tendency, but this power is unlocked and experienced by the user through their own faith and devotion to Sai Baba. The immediate success the gentleman experienced was a direct result of following this advice.