"Non-abiding mind" is the goal of Zen training. Thus, sitting resolutely with a non-abiding mind is both the practice and (if and when truly done without mind) also the enlightenment. The merging of everyday activity with enlightenment via no-mind is Zen.
When doing sitting Zen (坐禪) the training is to cut off all attachments, forget all views, drop thinking, and maintain the immovable mind.
Is this training limited to sitting meditation? Not at all. It is just that it is easiest to prevent your awareness from fixing on sense-objects or mental chatter when you are sitting resolutely with your back straight, so this is the preferred method of Zen yogins for attaining no-mind.
Let's say you were in prison and learned you'd been sentenced to death, and that your execution would happen at dawn. You could pace around the cell all night, write some eloquent letters protesting your innocence, start gibbering like a madman, or bash your head against the walls of your cell until you collapsed unconscious.
But the Zen way to deal with this situation would be to sit resolutely with your back straight and attain the mind that is neither inside nor outside.