At the very beginning of a player's round before he divides or moves any stack, the player can declare to become a resting player. He can only become a resting player, if one of his stacks is locked in combat.
A player can also be forced to become resting.
If the player declares that he becomes a resting player, his player's round stops immediately and he can not take player's rounds until the next set of battles is fought. A resting player's stacks are ignored for game purposes and can not be attacked.
Designer notes.
See resting.