[Idiom] the fins on the parallel lines

[vaibhavmaskar]

We were told that a strong attraction to a patient with a repeated history of failed treatment is a danger sign—like" the fins on the parallel lines." It is an illusion—a cognitive illusion—and I (System 2) was taught how to recognize it and advised not to believe it or act on it.What is the meaning of phrase in quotation mark?

 

[MikeNewYork]

My guess is that it refers to a drawing which was an optical illusion. Without the drawing, it is impossible to be more specific.