Currently I am reading about Heisenberg Group. And I understand that this group is one of the simplest examples of sub-Riemannian manifolds. I have read a lot about it structure, geodesics and e.t.c. Mostly from this Lecture Notes (attached below): https://cvgmt.sns.it/media/doc/paper/5339/sub-Riem_notes.pdf
But now I do not clearly understand, why we restrict the bundle and work on the Horizontal sub-bundle in case of the Heisenberg group. Why it is said that we cannot move directly along the Z-coordinate?
If you can share any materials or thoughts about this issue I will be very grateful!