It is pretty simple, Doug. In the cases I presented in the other discussion, the second Chain is simply an offset from the 1st. It maintains the x-direction of the sketch. It is a terrific way to assure the orientation of your newly created feature.
The problem comes when curves are highly dependent on how they are created to know what their orientation is. One of the worst conditions is when you create a cylindrical curve from equation, where the curve "twists" back to the CSYS it is defined from. If you use it for a spring with a round wire, you'd never know... but in my case, I used a square wire, and boom! Not acceptable!
So the re-definition, using the very same equation with a slightly larger R definition now guides the sweep with X pointing to the Chain (the second helix).
Now this is not without challenges, however. Continued efforts with this practice did have some odd behavior when the two came too close together. It actually jumps one loop and re-oriented the X at a diagonal. Easy to see, but if you have a very dynamic part (table driven or alike), you could end up with a bad instance. It doesn't necessarily fail, but it is not what you'd expect.
The take-away is that if you sweep a surface to create an edge, you might consider sweeping it to create 2 edges or intersecting a second surface. That way you have an X-direction for the secondary path (Chain).