The default state of a unit is at rest, but it stops being at rest when: It enters the realm, OR It moves or is moved, OR It casts a spell, OR It activates an activated ability or triggers a triggered ability. It returns to being 'at rest' after passive effects have been applied and the Storyline is empty. Here are some clarifying examples: If a Hillock Basilisk moves and attacks, the minions in the attacked location will be disabled before they have a chance to use their Defend triggered ability, because passive abilities apply before triggered abilities. But, a defending minion moving in from 1 step sideways will NOT be at rest because once that defending minion uses its triggered ability, it's no longer at rest. It won't return to being at rest until the whole attack sequence finishes. Also, it's doubly not at rest because it moved! Units being carried by other units will not be at rest while being moved. Units being forcefully moved by an effect like Grapple Shot or Windblast will not be at rest, since they're being moved.