Let us assume the American schoolroom convention for naming the three orthogonal lines that flesh out 3-space. For navigators, the X and Y spanned the ground—i.e., the surface of the Earth. X was left–right and Y was forwards–backwards. But in the classroom you learn X and Y first, and Y is vertical. We use that convention here. It is illustrated below:
The forwards–backwards (Z) axis really is only an inferred from zoom, which is magnitude along the X and Y axes. For example, the cognition distance away is directly proportional to shrinkage along the X and Z axes. Visual space arises originally as two-dimensional. That 3-D depth effect is really just a mismatch of X-positions on the two slides, along with shrinkage.
Thus changes that do not involve Z-wards motion yet signify that motion. This ubiquitous translation to and from the Z-axis is essential to the Salvia revelation. You realize that beholding in the forward-backward dimension is just one way of handling time on the occasion of proprioceptive self-motion.
Time and gravity are felt to be connected. When you lean forward on Salvia and your face rains the past downwards into the stream of arising-and-passing that flows in the direction of gravity, have you stopped to consider the relation of your now-awareness to those forwardly receding face-masks. Are they self? And should I try to force them to stay here in the present?