The aim is not to strengthen the extensor muscles but to make more skilful use of them, and to explore how the whole back can be responsive to our intention to look around the room while we're lying on the belly.
As you do this lesson, start gently, move slowly, and rest as often as you need. Emphasise curiosity over ambition; curiosity about new sensations, about how your intention to look around your environment can help you discover a more integrated, less effortful way of moving. Focus less on achievement and more on discovering ease, comfort and enjoyment.