Hakuin found Rinzai Zen in Japan in a poor state, with koan work reduced in places to the memorising of accepted answers, and he rebuilt it. His most famous single contribution is a koan of his own devising: two hands make a sound when they are clapped, so what is the sound of one hand. He introduced it as a first barrier because he judged the traditional opening koan, Zhaozhou's mu, had become too well worn to stop anybody.
Behind it is his doctrine of great doubt. Hakuin held that the practitioner must be brought to a condition he described as being frozen inside a sheet of ice or swallowing a red hot iron ball, in which the ordinary machinery of understanding is completely blocked and the question cannot be put down. Beneath great doubt lies great awakening, he wrote, and where there is no doubt there is no awakening at all. The koan is not a riddle with a clever answer; it is a device for producing that blockage reliably.
He was equally insistent on what comes after. He had contempt for practitioners who took one opening experience as the end of the matter, and he arranged the koans into a graded curriculum to be worked through for years afterwards, so that the insight was tested against every part of the teaching. Every Rinzai lineage in Japan today descends from him, and the curriculum they use is substantially his.