"Tension is created by opposition. In this case a curved surface bulges toward the straight line blocking its path. A still stronger feeling of tension is between the arrow and the perpendicular line."
Equal masses pressing towards each other force the space to flow around them. This kind of relationship we sense as tension. The opposition between the two arrows is easily felt.
When these forms touched the straight line, the sense of tension was lost. Although the rounded part of the shape is fixed to the straight line, they seem to be trying to move away from it. The arrows, however, seem to have their points embedded in the line.
Does anyone want to go through this book and do the exercises? Or Abstraction in Art and Nature?
These are the two best "advanced design thinking" books I've seen (that I can understand, at least).