Stability is usually the missing component for folks struggling with long-arm compression.
Stability of the scapula on the ribs, stability of the head of the humerus, stability of the upper back, stability of the pelvis, stability of the head of the femurs in the hip sockets... So much required stability!
This exercises emphasizes stability to focus on lumbar flexion. Relative stability is the idea that providing stability in one part of the body allows you to concentrate movement in another. In this exercise, relative stability in the shoulders, upper back, and hips allows you to articulate through the low spine.