Traditional fly casting instruction states that the line follows the rod tip. This video:
vimeo.com/aitorc/does-the-line-follow-the-rod-tip
shows how that idea is just a rough approximation to what actually happens.
One example of the limited value of that statement is bad tracking: if the back and the forward casts are separated by less than 180º in the horizontal plane strange things happen on the tip of the line. Why is that possible if "the line always follows the rod tip"?

In this new video example we can clearly see what happens when we have a faulty tracking:
1.- Since only part of the line moves in the direction of the rod tip motion a wave appears in the rod leg of the loop; that wave travels down the line.
2.-The tip of the line is put in motion in a different direction, and as Newton taught us, that length of line tries to keep moving in that direction due to its inertia. At a given time, the line and the leader are positioned 90º from each other. That's the explanation for the curve cast presentation that sometimes results from these faulty casts.

The same phenomenon (though in a more vertically oriented plane) is behind the problem of the "dangling end" that appears when casting medium to long lengths of line:
vimeo.com/aitorc/dangling-end-explained