Master Mumon went to a Ch'an temple and worked on Joshu's "Mu" day and night for six years under the direction of his Master.
What does this mean? It means that he worked every day with the other monks to grow food yet also spent 6-8 hours out of every 24 doing intensive lotus-posture sitting meditation in the Buddha Hall.
During his sitting meditation he "held" "Mu" in his mind, nothing else. If anything distracted him from "Mu" he summoned up all his energy to renew his concentration on it without falling into any thinking "about" it or any ideas or interpretations (such as, "Joshu must have really meant . . . " ). He turned "Mu" into an iron wall and gazed at it until his thinking-spirit was totally strained and nearly exhausted.
Then one day at dawn after sitting up all night in the Buddha Hall he heard the temple bell, or the temple bell heard Mumon, and inside and outside spontaneously unified into a single no-thing; everything in the world became as clear as a sheet of ice. His thinking had vanished. There wasn't a single idea in his head. His head wasn't even in his head. There was no more sense of a "he" to be in his head or out of it.
But even this wasn't the end. The sheet of ice, thick as a glacier, now shattered into a billion fragments. In a flash, beyond any words or ideas, he realized the infinite extent and depths of the "true Self-nature." He jumped up and began laughing and dancing wildly, and now he composed his famous enlightenment poem.