Rest

Rest is like "defense" or "toughness" in Yu-Gi-Oh! or Magic: The Gathering. At the end of each turn, whenever one of your cards loses any number of Power, it gains that much regardless of its actual Power stat. If you have 30 Rest, and you lose 20 Power, and your Power stat is 70, then actually you will have 80 at the end of your turn, but not be able to rest until you take another 20, then you would have 90, then 70, and the cycle continues, which is why you need to attack with multiple cards, regardless if they are killed or not.