In design sketching, it’s essential to know how to represent ideas fast, especially during the preliminary phase of research of a project. We are not yet looking for “sexy drawings”. We want to allocate more time at thinking, and multiply the number of sketches.
Less time we spend for one sketch, more sketches we’ll be able to produce. So instead of drawing with shadows (time consuming), we use the contour lines to express the surface of a volume.
Hope it helps, and see you for the next tip of the day !
PS: I did this tutorial on Photoshop with the Opacity and Size pressure activated. It gives a natural stroke to the pen.