This description of the Walk and Turn test posted by San Francisco DUI Lawyer.

In the absence of demonstration, the validity of the test is affected. The officer should make sure the right foot is in front of the left foot to start, for standardization purposes. This is important for the subsequent walking stage, specifically in the evaluation of the turn, so that a person is not "wrong footed" as it is commonly described. If the suspect is instructed or demonstrated improperly it may affect the suspect during this part of the test. After placing the person in the starting position, the officer then explains the test requirements, using verbal instructions and demonstration. 

There are two scoring factors that can assessed to the suspect's performance during the Instruction Phase. If the suspect cannot keep balance while listening to the instructions, that is when his feet actually break apart. This item is only scored if the suspect does not maintain the heel to toe position throughout the instructions. The officer is trained to be conservative in their scoring and not to score a point if the suspect sways or uses the arms to balance but maintains the starting position during this stage.