Hi mir4ea, that option would actually be better labeled something like "Move profiles on parallel planes".

It will currently only have an effect if there is no scaling rail being used, all profiles are open curves, and both rails are open and are made up of one single segment, and also all profile curves have to be on parallel planes.

When it is activated, instead of using the regular method for sliding the profiles along the rail curves, it instead moves along one of the rails and intersects the other with a plane (using the common profile plane normal) so that all the generated profiles for the sweep are on planar slices. In the default sweep mode when things slide along different length rails it will usually cause the profile curves to kind of bank around a bit instead of staying all in one plane, that banking gives a twisting effect and can make a sort of bulge near the ends.

In the future I think I'll be able to relax some of the restrictions on this mode and rename it to something about using parallel planes.

- Michael