Imagine a Rembrandt portrait with that "Rembrandt lighting", half the face in full light, the other half in shade.

With slide film you place the dome near the cheek in full light pointing it toward the camera. That gives you the incident "expose for the highlights" reading.

With negative films you place the dome near the cheek in shade pointing it toward the camera. That gives you the incident "expose for the shadows" reading.

My €0.02.