During Zazen, the conscious flow of thoughts is significantly reduced and the thinking brain is calmed. Blood travels to the deeper layers of the brain, the thalamus and hypothalamus, and this body-brain becomes more active and developed. When the deep brain becomes more active, the nervous system relaxes. By being receptive and attentive in every cell of your body, you learn to think with your body, unconsciously.
During zazen, thoughts, conscious and subconscious, naturally and constantly rise to the surface of our minds. Don't try to suppress this. But at the same time, don't get involved in the thinking process and don't let thoughts take you away from concentrating on your posture and breathing. Just let the thoughts float by like clouds in the sky, without resisting them or becoming attached to them. Shadows pass and dissolve. Images rise up from the subconscious and disappear.
The brain becomes deeply calm. We come to the deep unconscious, beyond thinking, to hishiryo consciousness, true purity.