The "koan" practice in Rinzai Zen is a key aspect of the teachings of Hakuin Ekaku, the Reviver of Rinzai Zen. Koans are paradoxical or nonsensical statements or questions that are used as a tool for spiritual growth and enlightenment. They are meant to challenge the rational mind and lead to a breakthrough in understanding.

In Rinzai Zen, koans are used as a form of meditation known as "koan introspection." The practitioner is given a koan by their teacher and is expected to contemplate it deeply, often for extended periods of time. The koan is not meant to be solved through logical reasoning, but rather to be experienced through direct insight.

One of the most well-known koans in Rinzai Zen is "What is the sound of one hand clapping?" This seemingly nonsensical question is meant to break the practitioner out of their habitual ways of thinking and push them towards a state of "great doubt" or "not knowing." This state of not knowing is seen as a necessary step towards enlightenment.

Koans can also take the form of stories or dialogues between masters and students. These stories often contain a twist or unexpected ending that is meant to jolt the student out of their ordinary way of thinking and bring them closer to understanding the true nature of reality.

The practice of koan introspection can be challenging and frustrating for practitioners, as they are often faced with seemingly impossible questions or statements. However, through persistence and deep contemplation, the practitioner can reach a state of "kensho" or sudden enlightenment.

Hakuin Ekaku emphasized the importance of koan practice in Rinzai Zen, stating that it was the "essential heart" of the tradition. He believed that koans were a direct path to awakening and encouraged his students to engage in koan introspection with great sincerity and determination.

In conclusion, the koan practice in Rinzai Zen is a unique and powerful tool for spiritual growth and enlightenment. Through the contemplation of paradoxical statements and stories, practitioners are able to break through their habitual ways of thinking and come closer to understanding the true nature of reality. This practice, as taught by Hakuin Ekaku, continues to be a central aspect of Rinzai Zen teachings and is still used by practitioners today.