Here is a detailed tutorial on constructing facings and understitching them. It has great line drawings that are easy to understand.
Understitching is a technique you can use on any faced opening, i.e. neckline, sleeveless armhole opening, waistband. It causes the facing to curl to the inside of the garment and gives a nice stable edge on which to press the seam to the inside. Understitching is a technique that gives a very professional looking finish.
Check it out: