I know of another reference to Zen Buddhism in Ajahn Chah's work:
(an excerpt from his talk, The Two Faces of Reality)
The Empty Flag
I once read a book about Zen. In Zen, you know, they don't teach with a lot of explanation. For instance, if a monk is falling asleep during meditation, they come with a stick and 'whack!' they give him a hit on the back. When the erring disciple is hit, he shows his gratitude by thanking the attendant. In Zen practice one is taught to be thankful for all the feelings which give one the opportunity to develop.
One day there was an assembly of monks gathered for a meeting. Outside the hall a flag was blowing in the wind. There arose a dispute between two monks as to how the flag was actually blowing in the wind. One of the monks claimed that it was because of the wind while the other argued that it was because of the flag. Thus they quarrelled because of their narrow views and couldn't come to any kind of agreement. They would have argued like this until the day they died. However, their teacher intervened and said 'Neither of you is right. There is no flag and there is no wind.'
This is the practice, not to have anything, not to have the flag and not to have the wind. If there is the flag, then there is the wind; if there is the wind, then there is the flag. You should contemplate and reflect on this thoroughly until you see in accordance with the Truth. If considered well, then there will remain nothing. It's empty, void; - empty of the flag and empty of the wind. There is no birth, no old age, no sickness, no death. Our conventional understanding of flag and wind is only a concept. In reality there is nothing. That's all! There is nothing more than empty labels.
If we practise in this way, we will come to see completeness and all of our problems will come to an end. In the great Void the King of Death will never find you. There is nothing for old age, sickness, and death to follow. When we see and understand in accordance with Truth, that is, with Right Understanding, then there is only this great emptiness. It's here that there is no more 'we', no 'they', no 'self' at all.