When you turn on Bend Links Mode (on the Bend Links rollout), rotating a single chain link causes the other links to bend following a natural curvature. When Bend Links Mode is off, rotating one link rotates all its children links by the same amount.
Bend Links Mode active (left) and Bend Links Mode inactive (right)
Tip: Bend Links works well on the biped spine, neck, tail and ponytail links.