To sit in the full lotus posture dropping off body and mind is to forget the self.
This is the true state of just sitting.
Usually when Zen people talk of just sitting, they are describing their experience of sitting as a blind physical act, an unconscious reaction to the thought of sitting upright. In other words, they confuse true just sitting (called SHIKAN TAZA by Zen Master Dogen), with the effort that they are quite properly taught to make, as beginners, on the basis of the physical conception.
The most fundamental teaching in Buddhism is to sit in the full lotus posture with the body, remembering oneself with body-feeling. This is the starting point, the foundation.
The opposite conception is to sit in the full lotus posture remembering oneself with mind-thinking. To sit like this is to perform a kind of somersault.
Thus can one lay the foundations for forgetting oneself in sitting itself. This is not to perform a somersault. It is to be somersaulted by sitting itself. To be swept into the vortex of the sitting biosphere. To become temporarily free of selfishness.