Understitching is a technique to help your facings lie flat on the innerside and hidden from view. It is used wherever facings and linings are required, such as on necklines, button openings, collars, inseam pockets, etc. In this article you will find resources to help you understitch.
Collen G. Lea of Fashion Sewing Blog TV, demonstrates understitching in areas beyond facings, such as collars.