We talk about zazen in lots of strange ways. We say, for example, that it isn’t the person practicing, but zazen practicing zazen, or Practice Buddha practicing zazen: the language is just an expedient means to drop off the primary dualism between self and world.
If the self can be dropped off, we can understand that there are two aspects to Impermanence, which correspond to Dogen’s formulation, in the Fukanzazengi, of zazen as the dharma gate of peace and joy.
The first is ceaseless arising and perishing, which is within our normal experience. We can understand and accept that this arising and perishing is the dynamic functioning of something whole, which we cannot see, as we are part of it, but if we only understand Impermanence from this perspective, then our practice is unbalanced: it is only the cultivation of equanimity, peace.
The second is the ceasing of this arising and perishing, which we can experience directly in zazen. It is as if each being-moment becomes like vast space, becomes like a mountain: it does not move, it does not flow. And this is joy.