Basically just experimenting with different tapers to do different things. I got the idea from an old-timer who taught me to make sinking lines that would keep you tight to your fly by sinking at different rates before you could readily buy them out of the box that way, and I’ve been experimenting off and on since. The one I’m looking to make with the 6wt is a line that will get down in a hurry but have a parabolic shape to it so that the fly will strip in at roughly the same depth the whole way back.
It's still in progress, but it’s akin to a booby line. Oversimplifying, but the tip/front part is a type 3 or so, middle part is a type 6 or so, and the running line is a type 3 or so. The idea is that the heavy middle pulls the fly down quickly, and holds it at/close to whatever depth as the line is stripped back in instead of having the fly climb.
Edit to add info/clarity: I have yet to see a saltwater-sized line (one that will throw saltwater flies) that does this, but the SA Stillwater Parabolic is close in concept to what I'm working on--just doesn't hold depth as well or throw bigger flies as well as I'd like.