The human body performs at its best when we strike the right balance between stability and mobility. The pelvis and lumbar spine, which is our centre of mass, need to be stable so that our limbs work off a solid well controlled platform. The trunk needs to be mobile to rotate well through the ribcage. The hips should have full external and internal rotation capability.
Often we end up being the reverse, too mobile through the pelvis and lumbar spine and too stiff through our ribs and hips. These are compensating strategies for a shortfall in core stability. To achieve your full movement potential, stability needs to be achieved where needed first to free up less mobile areas.
Image credit - LivePilates