>>7787437 (OP)
Baitpost, but serious response.
Some of posters here are definitely right, it's just visualizing the subject and its surface, not just the sillhouette but the "internal" area as well. How it has bumps and valleys, if something has weight, if the material is soft or hard and gives way etc. Even imagining what you cannot see, the back, thinking of the entire structure.
When you draw a line over and around the sphere, on paper you would represent it as just that, a line, if viewed head on. What you should instead consciously try to do is imagining that some areas of this line are located closer to the viewer while the rest is further away while drawing it. You can't actually physically move your hand back and forward, but you imagine it just the same. When you do an upwards stroke you are drawing the front and middle of the line is closest, butwhie doing a down stroke you imagine drawng the line on the back. Basically drawing an ellipse that is a line. This isn't some mumbo jumbo.
If you watch vilppu he makes a lot of aerial movements with his hands without touching the paper between strokes, this is him imagining a 3dimensional structure and drawing over and around it.
You can do targetted exercises to help develop this "sense" by simply playing with simple forms and manipulating them, gradually graduating towards more complicated interlocking forms. Just drawing crosscontour lines over pictures will help a lot.