Centralization and peripheralization of pain are ideas from Robin McKenzie, a physical therapist who specializes in neck and back care. Pain from a disc pressing on the spinal nerve can cause back pain that goes into the buttock and down the leg.
By performing specific motions the pressure can be taken off the nerve. The pain "retreats" or moves away from the foot and leg and occurs just in the low back area.
Peripheralization of pain means the pain gets worse and goes from the back into the buttocks and down the leg. Or if there already was buttock and leg pain, it gets worse. The goal is to avoid peripheralization and to centralize the pain. This is a sign that the disc is moving in the right direction toward healing.
The therapist will help you find movements that centralize pain. These will become the exercises you'll do throughout the day. She will also help you notice which movements peripheralize pain. You'll want to avoid those positions and motions.