Eldrad Ulthran

After both sides have deployed at the start of a game, the Eldar player may reposition D3+1 units in his army. No unit may be moved outside of its deployment zone.

This special rule is automatically lost by an independent character joining a unit that does not have this special rule, and vice versa.

There are many variants of this rule: Fleet of Foot, Fleet of Claw, even Fleet of Hoof. Title aside, all models with these abilities are treated similarly. In a turn in which models with this rule do not shoot or use a psychic power that replaces shooting, they can move an additional D6" in the Shooting phase, ignoring penalties for difficult terrain.