Even though I have received a following questions from Ven. silentbell San, but I have failed to answer to him, and so I would like to send my answer by this comments.
During zazen what is it exactly that I am supposed to be concentrating on? Do I try to "feel" my body sitting straight, and bring my awareness back to my posture again and again? Or do I focus on any stillness that might appear between my thoughts? Or do I just sit not focusing on any thing, just letting everything come and go? Thank you again so very much for the teaching you offer.
Gudo's Answer : First of all we should make our efforts to keep the lower spine straight vertically, and then we should keep the backbone straight upwards as far as possible. Pull the chin downward and backward to make the back of neck as far as possibly straight. Master Kodo Sawaki taught us that we should stretch our spine as if it were possible for our spine to pierce the ceiling actually. By stretching the back of neck our sight might be directed about 45 degrees downward.
During Zazen it is the most important matter to keep the posture straight vertically. It is never only feeling, but it is just the real Action. It is never the feelng, but Action itself. Therefore it is not awearness, but Action itself.
Therefore it is the most important matter for us actually to sit in the Actual Posture, not the mental consciousness or sense perception. Zazen is just Action, and so it is just the traditional posture.