Think of not-thinking. How do you think of not-thinking? Be before thinking. These are the basics of zazen.
When one's perceptional faculties are acutely developed and on the cusp of awakening, logical perception will begin to occur before emotional perception.
For example, if there is a loud bang, the first perception in the mind will be, just a loud bang, nothing to fear. The very next nano second, the emotional perception will arise, suggesting fear and panic. However this emotional message will have no relevance because the event has already been cognized logically.
In modern scientific terms, one could describe it as the speeding up of the logical brain over the pre-brain.
By intently examining every moment in zazen we cause the evolution of the response time of the brain's logical faculties.
To be before thinking is indeed a very literal action.
The pre-brain, or animal-brain, or Amygdala evolved in human evolution as a primtive information processor, only emitting a binary response - fear or pleasure. It is very stupid but very quick. It keeps one alive, uses very little energy, but is useless for day to day human activity. The pre-frontal cortex is a far more sophisticated organ, whose response to inputs includes no emotion, only logic, but takes more calories, and has a slower response time.
In a normal unenlightened brain, the amygdala responds faster than the PFC. By the time the PFC gets event data it is already meddled with by the Amygdala and has been characterized as positive or negative.
To attain being before thought we train in faster response and intelligent reponse.
When we do long hours of zazen, by choice-less-ly observing events, we strengthen the nervous system leading to the pre-frontal cortex, and weaken the nervous system leading to the amygdala. Exactly like strengthening a muscle with use, and atrophying another muscle with disuse. In time the PFC reacts faster than the Amygdala to everything leading to a very logical person who's not swayed by the emotional ups and downs of life.
However, this is a physical modification, and can change with age or injury.
The real permanent change is when we also train our logical faculties to learn to recognize and discard the emotional meddling of the amygdala if and when it arrives before the PFC.
Thus even if an event is characterized as fearful or anger inducing by the Amygdala, by realizing no-self we can discard animalistic habit patterns, and utilizing the energy of mindfulness and wisdom we decide on the moment on its merits.
Eventually after practicing this until it becomes second nature, and by discarding identification and habit patterns we are finally before thought, and the turbid waters of the mind become still and clear.
CAVEAT: I am not a neural expert, this theory is solely based on personal inquiry, a little experience and a little hypothesis. YMMV.