Kobun Chino Otogawa had this to say about shikantaza:
Shikan taza sounds very strong. Shikan is understood as identical to taza. Shikan means “pure”, “one”, “only for it”. Ta is a very strong word. It shows moving activity. When you hit, that movement is called ta, so “strike” is ta. Za is the same as in the word zazen, sitting. To express the whole character, shikan taza is actually quite enough, but not enough until you experience it. Shikan taza is sitting for itself. You may say pure sitting for itself, not for something else.
Shinjin datsu raku is the same as shikan taza. Shinjin is “body/mind”. Body/mind is nothing but our whole life. This cannot be seen in two ways; body/mind is one thing. Datsu is “to refrain”, and “to drop from”. When you are dreaming some terrible dream, and the dream is cut off, that is called datsu. When you get rid of that dream, that also is called datsu. When you have a sword, the action of pulling a sword from its sheath is called datsu. So datsu has a very strong meaning of freeing from something. Another way to express it is : to have conquered something which hindered your existence, like attachments, delusions, or misunderstandings. Zazen itself is cutting off those conditions.
If you find body/mind as one thing, you have only found where you are at this moment. When you find where you are at this moment, you will lose where you are at this moment unless you allow the sense of where you to shift and move, to have a freedom. This is how we fall asleep. This is how we wake up.
Any posture that is held for a period of time can allow for the observation of reciprocal activity related to pitch, yaw, and roll, as a matter of necessity in the relaxed movement of breath. I find pitch, yaw, and roll is actually present in the experience of location itself, along with the necessity of a freedom of sense. When I realize the necessity of a freedom of sense, I lose the doer, yet it’s the necessity and not the realization that acts.