Under-stitching assists a facing or lining to stay to the inside and un-seen. It is stitching that is sewn as close to the seam line as possible holding the graded seam
allowance to the facing or lining.
Also Known As: stitch in the ditch
As long as I grade the seam and under-stitch the facing, the facing will always stay inside the garment and not "roll" to the outside.