When you open up the back of the heart and across the chest, you create space for the heart, which is both valuable and liberating!
When you are practicing backbends, you must try to distribute the space as evenly as you can throughout the spine, and focus on the right part of the back. The lumbar area of the spine, the lower back, is usually very flexible, and we generally need more strength here than space. The same goes for the neck.
Try to focus your backbends around the part of the spine that needs the most opening: the very back of the heart, the thoracic spine. That's why it's called a heart opener — we actually open up right at the center of the heart.
Editor's note: Warm up your spine with a few rounds of Cat/Cow postures before you begin.