Hi JTB,

That is for adjusting what is called the "seam" of the surface.

Basically when you are lofting closed curves, MoI has to decide how to align each section with the next section. It does an automatic calculation to minimize the twisting between sections, but it allows you to adjust the alignment of each section to fix it in case it did not do a good job with the automatic calculation.

However, you can also use the adjustment to purposely introduce a twist if you want!

You can get the equivalent of this for extrude by making a copy of your curve and then doing a loft between the 2 copies.

- Michael