It is a little more complicated than that. It remembers the last formation where the player was attacked or attacked another player from the same Realm. If you are trying to set up for HP the best thing to do is sometime after Cerb on Friday but before the Lady have the first and 2nd strip down and hit each other. Then after that no one from the same Realm can attack them in the Arena and they can not attack some one from the same realm in the Arena until server reset. Note that after the set up is done they can power back up and hit any one in the pond or the wilds or anyplace else except the Arena.