According to the provided text, what is the role of a Guru in understanding profound spiritual texts like the Upanishads?

The provided text from Chapter 20 emphasizes that a Guru's grace is indispensable for understanding profound spiritual texts. It explains that the Upanishads are a treasure full of deep meanings that are 'very difficult to grasp' without the grace of a Guru. For a person who is attached to the Guru's feet, the text states there is 'not even a bit of difficulty.' The Guru's presence helps the hidden meaning open up automatically, the 'obstruction of the intellect goes away,' and clarity is achieved. This is presented as the only way to access the science of Self-knowledge, which is described as a weapon that cuts through birth and death.