... actually, it's just experience, not sorcery. The quality of your line is consistent with what comes out when you are trying to draw a long line with a writing motion. Add to that the sad fact that most people are taught to write (somewhat) but next to none are taught to draw.
If you have trouble switching to the proper arm motion with a pencil, get some charcoal and practice drawing long lines on a big piece of paper - one a few feet tall. Memorize the motion that you have to make to get a continuous line with those.