[Freesurfer] White matter editing using control points

Hi,

I am having some trouble with white matter editing using control points. I am placing control points in white matter with intensity <110, and I find that it seems to have different effects in different slices/subjects, which makes it hard to develop a general standard for control point editing. Additionally, the curve outlining white matter is often less correct after control point edit, also some distance away from where control points are added. So, how does Freesurfer interpret control points and what exactly happens when control points are added? How does the number of control points used and the intensity of the voxels in which the control points are placed determine the outcome?

Thanks,

Jannike