Regardless of the type of line you’ve installed, you’ll almost certainly find that it will slacken over time.
Learning to tighten the rope around a rotary line or a retractable line are broadly the same in terms of technique. It’s simply a case of choosing the best position to apply your tightening technique in order to counter the stretch that results in line droop. After tightening several times, you may find that your line begins to weaken, and eventually the plastic cable or rope will need replacing in its entirety.
Here’s a video to help visualise the techniques.