Pre-backflip Exercises 

If it’s still relevant: You want to ensure it’s the lower back because that allows you to “compress” your core. Look into a “hollow body hold” and see how it feels. Most people suffer there and compensate by extending at the lower back, creating enough anterior tilt in the pelvis to allow for a good external rotation. Essentially, we cheat by taking out the deeper core and hip muscles, and instead compensating with other movements. Keeping your lower back in contact helps prevent that cheating. If it’s hard, that’s not uncommon for muscle groups we aren’t used to using. You can regress to bent knees if it helps initially. I hope that helps!