C-x following by the UP and DOWN arrows switches between coordinate systems.
Starting from the “cartesian” coordinate system, typing C-x down will select a “structural” coordinate system, and if your current dimension was the second one, “lines”, you should now be in the second of the structural coordinates, which is “exprs”, that is, “expressions”. The minor cursor keys will now select previous and next s-expressions (you will see the current expression is highlighted), and the major cursor keys will select the depth (you will see the pair of brackets highlighted – Versor can use multipart selections).
You can control which dimensions are available in each mode (see Dimensions available).
You can also put these commands onto M-up and M-down by
putting 'meta amongst the arguments to versor-setup.