In Rinzai Zen, koans are an essential part of the practice and are used as a tool for achieving sudden awakening or enlightenment. Koans are short, paradoxical statements or questions that are used to challenge the logical mind and encourage the practitioner to look beyond conventional thinking.

The practice of koans in Rinzai Zen is known as "koan introspection" or "koan study." It involves the student working closely with a teacher, known as a roshi, who assigns specific koans for the student to contemplate. The student is expected to deeply reflect on the koan and present their understanding to the roshi in a one-on-one meeting, known as dokusan.

The koans used in Rinzai Zen are often based on dialogues between Zen masters and their students, as well as stories and anecdotes from Zen literature. Some popular koans in Rinzai Zen include "What is the sound of one hand clapping?" and "What was your original face before your parents were born?"

The purpose of using koans in Rinzai Zen is to break through the limitations of the rational mind and experience a moment of sudden enlightenment or satori. The koan is not meant to be solved through intellectual analysis, but rather through direct experience. The roshi may use various methods to help the student break through their conceptual thinking, such as shouting, hitting, or asking unexpected questions.

Koan practice in Rinzai Zen is a rigorous and intense process that requires dedication and perseverance. It is believed that through this practice, the student can reach a state of no-mind, where they are free from the constraints of the ego and can experience their true nature.

Koans are also used as a means of testing the student's progress on the path to enlightenment. If a student presents their understanding of a koan to the roshi, but it is deemed incorrect, they may be given a different koan to contemplate. This process continues until the student reaches a satisfactory understanding and is able to experience sudden awakening.

In conclusion, koans are a central aspect of Rinzai Zen practice and are used as a means of breaking through the limitations of the mind and experiencing sudden enlightenment. Through working closely with a roshi and deeply contemplating koans, students can reach a state of no-mind and awaken to their true nature.