This is not obvious to everyone, and most people have never observed a trolled line underwater.

Many graphic renditions and diagrams on the web get this wrong, even some who should know better.

Some assume that a line is straight, or close enough to straight.

Some have the first impression that a line has a trailing far end so that the line angle is steepest near the boat, and the end of the line is dragging on the ground.

In fact, a trolled line has a belly so that the line angle is steepest near the lure where the dive lip or its weight pulls downward, and the water blows the middle of the line back and up. To use your own real world experience, imagine holding a length of string with a small weight at the far end in a breeze.