I tried Nishijima's method the very first time yesterday. The method as I understand is as follows

It is "just sitting" as an ongoing action of "keeping the spine straight". As long as we are aware, we keep trying to keep the spine straighter and straighter; even when it is already straight. Whenever we wake up from thought we go back to that process again. I think it is pretty similar to what is described in "Opening the hand of thought" as "Aiming at coming back to straight line ZZ', waking up from drowsiness and distraction". But I like Nishijima's one better because it makes just sitting an active process as we are doing a real physical action.

Few observations. When I tried this method, my awareness was much much better compared to any other methods that I tried previously. At the same time, I felt the method is intense and when I sat in the weekend treeleaf Zazenkai, the second sit seemed to cause some pain in the neck and upper back area during the sit. Despite liking the method, I am afraid doing this incorrectly can hurt the spine.

Would like suggestions from those who followed this method.