Yeah the documentation isn't super clear on what torsion is. But from intuition, the torsion seems to be a derivation of the normal vector along the curve/surface edge/isoparm.
Longer quills = more torsion = curve is less planar in local area.
Shorter quills = less torsion = curve is more planar in local area.
Please don't quote me on this and verify within the software.
