It is common in Zen to say that shikantaza – just sitting, choiceless awareness – is not practice.
We are not practicing in preparation for anything, or to get somewhere. Shikantaza itself is the real thing. It is what we are noticing itself.
It is awakeness noticing itself. This timeless now within, to and as which everything happens.
In that sense, shikantaza is not a practice.
Yet there is also a practice element in shikantaza, which shows up in two ways.
First, it is the practice of shifting into what we are noticing itself.
Attention is absorbed on the inside of thoughts, it is noticed, and there is a shift back into just sitting. This practice happens on the cushion, often several times during a sitting period.
And this practice on the cushion is also a practice for daily life. We practice shifting into what we are noticing itself on the cushion, and then bring it into daily life.