From the book, Questions to a Zen Master, by Taisen Deshimaru:
In zazen, concentrate on your posture and let everything else go by. After a while, what is in the subconscious rises to the surface, because when conscious thinking stops, the subconscious mind can be expressed.
Freud and Jung wrote about it. Jung was a profound psychologist. He studied Zen, which he knew through the books of D.T. Suzuki. But he had no experience of zazen, and it is impossible to understand if one does not practice.
If you practice zazen you can understand the subconscious coming back to the surface. You must let it come up, and in the end it wears itself out: one year ago, five years ago, when you were a baby. And you get back to what is original, to complete purity. That is satori. Not a special state, not a condition of transcendent consciousness.
During zazen you must let everything go by; but willing yourself not to think is also thinking. Let your thoughts go, do not follow them.
“Zazen” refers to the traditional seated Zen meditation practice.
Back in the 1990s I bought my first three Zen books at a local Barnes & Noble store, and this was one of those first three books. Back then it felt like there were only about ten total books on Zen and mindfulness, but of course today there are hundreds of books. This book is very different from the more modern literature, and also very different from any sort of mindfulness book. I still enjoy going back to it from time to time, as I am now.