This line is a line of constant pressure (isobar) since shock waves cannot exist across isobars and for a well-designed wing coincides with a constant percent chord[6] as shown. The triangles show that only part of the incident airflow (in a streamwise direction) is responsible for producing lift or causing shock waves (ie that part shown by the arrow perpendicular to the red isobar). Its length behind the shock is shorter signifying that the flow has slowed down in going through the shock.