The Transverse Abdominis (TVA) muscle is one of, if not the most important muscle in maintaining spinal health and healthy movement patterns.
Running horizontally across the abdomen and hugs, hence the Spanx, inwards towards the spine. It is, or should be, engaged anytime a limb moves. Proper engagement acts to stabilize the spine and pelvis BEFORE movement, and helps to keep the spine (including neck) in a neutral position during training or movement.
It also helps to improve posture, balance, back and neck pain, postpartum diastasis recti, and weakness from prior abdominal surgeries. Pretty much ANYBODY with a spine should be aware of how to tap into these muscles. The above video is a brief tutorial on the tip of the iceberg of TVA engagement, breathing exercises.