Create a path, two points is a great way to explore this feature. Use the pen tool and hold down and create a straight path
Go to that panel in the control bar with the path selected and click and you will see the width, set it to something like 100pt and then go to the button / display at the bottom of the panel and click and you will see a grid. If the path is 100pt then each vertical bar is 25pt, so 0, 25, 50, 75, 100. The bars from left to right match the path selected so the far left matches the start point, the last right matches the end point. The dash feature cannot be used with this feature. Youtube video on subject
You get two, one at the start and one at the end. They are linked, so if you lower one to 75% then the other also moves. There appears to be no unlink feature.
You can add and delete (select and delete) points to the curve and you can drag the curve up and down in different points and if a path is selected, you will see it change. Place the mid point down at the bottom of the panel and it will be 0pt, place it halfway and it will be 50pt (if 100pt is being used). You can add multiple points. You can drag the point close together, have one point and one low to create a cliff effect etc
You can click the save and you will see the entry stored and you can then re-apply it to another path. It is saved only for that document. Open a new one and you will not see the entries though if you copy a path from one to the other and then click save, you can then use it.
An endlessly flexible line that can be modified at any time and can be combined with others to create all kinds of unique designs