Hakuin Ekaku is known as the "Reviver of Rinzai Zen" for his significant contributions to the revitalization and popularization of the Rinzai school of Zen Buddhism during his time. He is regarded as one of the most influential Zen masters in Japanese history, and his teachings continue to influence practitioners around the world.

Hakuin was born in 1686 in the small village of Hara, Japan. He was raised in a family of Zen priests, but he did not initially show much interest in Buddhism. However, at the age of 14, he had a profound spiritual experience that led him to enter a Zen monastery and begin his training under the guidance of the Rinzai master Shoju Rojin.

During his early years as a monk, Hakuin became disillusioned with the state of Zen practice in Japan. He noticed that many monks were more concerned with ritual and ceremony rather than true spiritual awakening. He also observed that the teachings of Zen had become diluted and lacked the intensity and rigor that was present in the teachings of the past masters.

With a deep sense of determination and commitment, Hakuin set out to revive the true essence of Rinzai Zen. He devoted himself to intense meditation and rigorous training, seeking to experience the same awakening that the Zen masters of the past had achieved. Through his diligent practice, he gained a deep understanding of Zen and its teachings, and he began to share his insights with others.

Hakuin's teachings were characterized by a direct and forceful style, which was in stark contrast to the more passive and intellectual approach that had become prevalent in Zen at the time. He emphasized the importance of intense meditation and the use of koans, or paradoxical riddles, as a means to break through the barriers of the mind and experience true awakening.

One of Hakuin's most significant contributions to the revival of Rinzai Zen was his emphasis on the practice of "koan introspection." He believed that the key to awakening lay in the direct experience of one's true nature, rather than in intellectual understanding. He encouraged his students to engage in intense introspection and to use koans as a means to break through their conceptual thinking and experience the true nature of reality.

Hakuin's teachings attracted many students, and his reputation as a master of Zen spread throughout Japan. He established several monasteries and training centers, and his teachings were passed down through his disciples, ensuring the continued growth and popularity of Rinzai Zen.

In conclusion, Hakuin Ekaku revived Rinzai Zen by emphasizing the importance of intense meditation, the use of koans, and the direct experience of one's true nature. His teachings brought a renewed vigor and intensity to the practice of Zen, inspiring many to seek true awakening. His legacy continues to live on, and his teachings continue to guide practitioners towards enlightenment.