Incident meters, pointed from subject towards the camera, give perfect meter readings for reflective surfaces which observe the Lambert law. As it just so happens human faces are sometimes more specular than the perfect diffuse reflector, and that's where incident meters may end up reporting incorrect values, i.e. highlight sections burn out. The method of pointing the incident meter at the light source is a crutch: of course it will show a higher EV that way and avoid burned out highlights, but simply applying an educated exposure compensation does exactly the same, and probably better.