How to make a “spreading effect” (appearance) of a line (image), but without its movement?

Question

Animation

There is a catch line, and you need to make it appear so that the line diverges in both directions, eventually becoming a full line. But the point is also that the "file" (line) is already complete.

I did it with an example clearly, but I’m still not familiar with the effects ...

Answer 1

I don’t know what those people write to you above in the comments, but the question is primitively clear.
No scale is needed here.
If manually
Put your line and make it a mask. Center the mask and make it zero width. This is your 1st mask keyframe. The second keyframe of the mask will no longer have zero thickness, but the one when the entire line is visible (you stretch the mask shorter in width).
It's a normal mask job.