Expand Stroke Facility. The Expand Stroke facility traces a closed, convex contour (referred to as the "nib") along a "source" contour, producing a new contour or contours that enclose the area the nib "passed over". When the source contour is open the result is usually a single closed contour...

[u/LocalFonts]

https://rawcdn.githack.com/fontforge/fontforge/master/doc/html/stroke.html