If you have modified a spline via the Modify->Drag function, or via other dimensions and constraints that change the relative position of its fit points, it can introduce a strain in the spline that may lead to bulges or even loops. This option will re-fit the spline through the current locations of the fit points, modifying the parameters on the spline where they are located, as well as parameters related to all other constraints and dimensions to the spline. This relieves the internal strain on the spline and allows for a smoother curve, avoiding the bulges and loops caused by internal strain. Re-fitting the spline will generally change one or more of its control point positions.