Ganhwa Seon

What is Ganhwa Seon?
Ganhwa Seon看話禪 (C. Kanhua Chan), commonly known as "questioning meditation" in the West, this meditative practice was systematized by Song-dynasty Chan master Dahui Zonggao, who taught students to use a Gong'an (空案 Kōan) as a "topic of meditative inquiry" (話頭 Hwadu).
In Japanese Zen, due in large part to the efforts of Hakuin Ekaku and disciples, Kannazen (Ganhwa Seon) became widespread within the Rinzai Zen tradition, where it was incorporated into an elaborate system of Kōan training, involving the systematic investigation of many different kõans.
However, in Korean Ganhwa Seon, the "topic of meditative inquiry" (Hwadu) must be examined with desperate doubt to know an answer to the inquiry rather than interpreting it from purely intellectual or conceptual perspectives. In particular, Master Subul argues it is a dead practice if the doubt can not be a main driving source to lead the practitioners to gain awakening during the Hwadu practice. He also stresses that there is only one-lifetime Hwadu to find an answer, and there is no need to examine one after another. One is solved thoroughly, and the rest are instantly resolved. That is the essence of Ganhwa Seon.
Ganhwa Seon continues to be the most common contemplative technique practiced in Korean Buddhism. Master Subul Sunim's greatest contribution to Ganhwa Seon is to reinterpret and re-introduce this magnificent ancient meditation technique with modern language, making it approachable as a daily practice for anyone.
Reference pp.415-416, Kanhua Chan from Princeton Dictionary of Buddhism
Don’o (Sudden Awakening)