The whole thing with bending lines in pre-fab lengths is that the tool for it prevents the sides of the tubing spreading out at the bending point. To crimp and flatten - the tubing has to widen out, so this makes a nice smooth bend that has no reduced flow.

Shown below is one kind of tube bender, they are all similar. Notice the grooves in the bending head - when the line is in these grooves, it can't crimp or flatten out, so the bending is done to the full line, it can't form a pinch point because the grooves won't allow it.

Each groove is a different width for diferent guages of tubing or pipes.

On this one, there is an adjustable bolt to bring the grooved head in to the rollers. Many tools that do this are much simpler, but this is typical of what you want in order to do free-hand line forming.

They usually cost very little