definition
An over the shoulder shot includes one person's shoulder on the edge of the frame.
director's intention
The director will use this shot to show the audience the action from the point of view of the character whose shoulder is seen. In this shot Voldermort sees Harry's fear and defiance.
impact on audience
The audience understands two things from an over the shoulder shot. The first is how the other character is seen from the point of view of the first character (whose shoulder is shown). Secondly, this type of shot shows clearly the emotion on the face of the second character. In this example, Harry is clearly fearful.