In Beyond Thinking a small book of Master Dogen's essays, Norm Fischer in his forward writes about thinking, not-thinking, and non-thinking. This is often confusing or taken out of context, these terms do not mean we must not have thoughts when we are sitting zazen.
When we say thinking, we are speaking of the mental process that generates thought. When we say think non-thinking, we are saying a thought will appear, but you don't have to think about it by adding more thoughts to the first thought. Then not-thinking refers to moments when thoughts are not being generated.
When we enter the silence and stillness of zazen posture it is a homecoming of sorts. The body is aligned naturally in a field becoming void of distractions. This field is unfolding differently than one in which we are encumbered. Breathing in is the beginning of life's moment and exhaling is expelling the toxins thereby providing room for the oxygen that sustains us in the next inhale. Breathing in then out and in again is a moment of life. This event horizon does not require thought.
When sitting zazen we are just being the face before our parents were born. Awareness is subtle not formed as in thought, more ethereal or zephyr-like. It is here we seem to want to rush into the void as if we fear or at least are uncomfortable with the loss of form. Returning to just being alive we don't attach to our pattern of thinking about what is happening but rather vaguely seem to witness the event.
Then, as we continue to sit still and quiet, thinking down-shifts or settles.
A thought appears but is not engaged therefore not gaining any traction disappears. Non-thinking gaps seem to appear in our awareness or habitual clusters of thoughts. Coming full circle now we see the point. When all senses settle into a natural state, they are without form. That is they have the capacity to see, hear, touch, taste, smell, and think but are not clinging to past events...they have exhaled and not yet inhaled...