Upanishad says well thought out teachings, we take the teachings and they think through it well. Clarity dawns in the intellect at that time. So there is conviction, there is clarity, there is tremendous change in our perspective. This is called Brahma Kaaravriti, modification of the intellect in the form I am Brahman.
Swami Sarvapriyananda teaches the Katha Upanishad, one of India's most revered texts which is also a part of the Vedas. It is a collection of philosophical poems representing a conversation between the sage Naciketas and Yama (god of death). They discuss the nature of Atman, Brahman and Moksha(liberation).