For subtle realization, it is of the utmost importance that you cut off the mind road.
It is easy to misunderstand the phrase “cut off the mind road.” Wumen is not asking us to stop having thoughts, but to stop following them. To stop following thoughts resembles Dogen’s advice: “to study the self is to forget the self.” When we forget the self we stop putting a construction we call the self at the center of our lives. Similarly, when we watch the pattern of thoughts that arise moment after moment we can follow them to their origins, which turn out to be nothing more than fantasies, constructions of the mind.
Seeing through these fantasies and constructions, we discover a world beyond thought, in which rain is only rain, not words or stories about rain. We come back to our true life, our true self.
The “subtle realization” that Wumen mentions here is nothing more than this recognition of our naked, unborn self, alive to this moment, alive to the world as it is, not as we think or construct it to be. This smell, this taste, this touch, sight, sound (with no description in the way), this life, in this moment, and we along with it—perfect and complete.
Blacker, Melissa; Ford, James Ishmael (2011-03-17). The Book of Mu: Essential Writings on Zen's Most Important Koan (p. 144). Perseus Books Group. Kindle Edition.