The Tai-Chi teacher Cheng Man-Ching mentioned an ancient Chinese description of meditation in one of his books: straighten the clothes and sit precariously.
When I sit, I remind myself that the two respirations (pulmonary and cranial-sacral) utilize the occurrence of consciousness to coordinate the activity of posture, and that relaxation and calm can allow the sense of location in the occurrence of consciousness to open nerve-connections to the surface of the body.
The activity generated by the sense of location in the occurrence of consciousness is involuntary, and the more I relax and accept the activity, the more precarious my posture feels. This for me is the meaning of the saying.