Rinzai Zen (Rinzai-shu)
The Linji lineage was first transmitted to Japan by Myosan Eisai. He initially studied at the Tendai headquarters on Mount Hiei to the north of Kyoto, but in 1168 he embarked on a journey to China, which culminated in him bringing Rinzai Zen Buddhism to Japan.
While the heart of the Soto school is based on the practice of Zazen, the heart of the Rinzai school (臨済宗) focusses on the use of koan, a kind of absurd phrase or statement which is given by a teacher to a disciple to trigger Enlightenment. In the Buddhist community, Rinzai-Shu is considered further from the Buddha's teachings than Soto-Shu.
Zen teachers often recite and comment on koans, and some Zen practitioners concentrate on koans during Zazen meditation. Teachers may probe students about their practice by using koan "checking questions" to further validate their experiences of awakening.
Designed to force and shock the mind into awareness, the koans are used to test the student's ability. While in the process of answering a koan, one comes to experience the koan as the mind itself, transcending dualistic thinking.
A student might work with one koan for several months or even years, returning to their teacher many times to comment on their koan. Some other students may require only a few seconds to 'understand' the same koan. Neither way of working with a koan is better than the other.
The most important thing about koans is their use as a tool to discover one's own true mind. It is not necessary to pass through as many koans possible.
Learn more about Rinzai-Shu: Rinzai Zen Official site
If you're seeking to explore Zen or Buddhism more deeply, here is a list of my favourite Buddhist books that you can use to learn more about this ancient tradition.