What is the key to understanding profound spiritual texts like the Upanishads, according to the provided scriptures?

According to the teachings in Chapter 20, understanding profound spiritual texts like the Upanishads through personal effort and intellect alone is nearly impossible. The text states that the Upanishads are the essence of the four Vedas, but this treasure is very difficult to grasp without the grace of a Guru. It warns that even a great scholar who tries to comprehend them independently will fail, as the path is difficult at every step. The true key is devotion and humility towards a Guru. For a person attached to their Guru's feet, the hidden meaning of the scriptures opens up automatically, and the obstruction of the intellect is removed, leading to profound clarity. This science of Self-knowledge is reserved for those who are humble and detached.