You do "an about face" when you begin facing one direction, then you turn completely to face in the opposite direction. Example: "If you ever discover that you are walking down a dangerous street, it is best to do an about face and walk in the other direction." You "do an about face" when you stand facing north, for example, then turn your body in one step until you face south. The term "about face!" is used in the military: The drill sergeant shouts "About face!" and all of the soldiers turn in one step to face the opposite direction. Example: "I want you to do an about face, get back in that bathroom, and brush your teeth!" To "do an about face" is also to change your position on something and take the opposite position.

more info:
goEnglish.com
http://www.goenglish.com/AboutFace.asp