Shobogenzo Dōtoku VIII: Hut Mastery.

Dōtoku continues with this old account of a hermit from Zen tradition:

In the order of Great Master Shinkaku of Seppō there was a monk who went to the edge of the mountain and, tying together thatch, built a hut. Years went by, but he did not shave his head. Who can know what vitality there was inside the hut?—though circumstances in the mountains were desolate indeed. He made himself a wooden dipper and he would go to the edge of a ravine to scoop water and drink. Truly, he must have been the sort who drinks the ravines.

Master Nishijima notes that 'drinking ravines' refers to the directness of the written version of the line 'going to the edge of a ravine, he scooped it and drank' which appears in the Chinese written source for this koan.

As the days and months came and went like this, rumors of his customs secretly
leaked out. Consequently, on one occasion a monk came to ask the master of the
hut, “What is the ancestral master’s intention in coming from the west?” The hut master said, “The ravine is deep so the dipper’s handle is long.” The monk was staggered. Without doing prostrations or requesting the benefit [of further teaching], he climbed back up the mountain and told Seppō what had happened. When Seppō heard the report he said, “Wondrous! Even so, this old monk will have to go and see for himself. By testing [the hut master] I will grasp [his situation] at once.” Seppō’s words mean that the excellence [of the hut master’s expression] is so excellent as to be wondrous, but the Old Monk himself had better go and investigate.

Master Dogen is simply explaining the Chinese characters of what has been said before at the end of this section. In response to the famous old question as to the ancestral master's (Master Bodhidharma's) intention in coming from the West (what is the essence/meaning of Buddhism?... in other words) the hut master simply replied with an appropriate practicality from his own life regarding his water dipper. The monk and Seppō are very impressed by his natural, straightforward attitude...