Imagine that our bodies are not actually just a front and back, but are in totality spheres; vortices actually. We are beings that are built around a central axis, exactly like our earth.
The gravity line or 'z-axis' is the line that runs through the center of our bodies. Orientation around this axis is critical to our alignment and thus essential to the optimal function of our bodies, our senses and our thinking minds. In ancient systems such as the Hindu Yogic philosophy, this line is called the sushsumna, in Taoist philosophy it is called the chong mai.
The gravity line explains the key foundational origins of our 3 dimensional presence. This line is actually not a line, rather, it's an axis that is formed by the 3 planes that comprise our environment, the space in which we move about. This axis is our point of orientation and integration, where the yin~yang expression experiences fusion and our bipolar nature stabilizes itself into a fluid and dynamic exchange of balance rooted in presence.