The eye of the observer can specifically be led across the image by varying the pressure on the pencil. The more accentuated the lines are, the more intresting and lively is the area in the image.
The accentuation of the stroke is caused by the modulation of line width and brightness, by giving more or less pressure on the pencil. Due to these differences, the drawing looks vivid.
Accentuated lines lead the eye of the recepient.
Sketch&Draw is a drawing technique by Tanja Hess http://www.sketchanddraw.com