Get high scores on all burial chamber battles located in the Labyrinth of Amala. There are ten burial chambers and 24 battles (tombstones) overall. You can check if you have a high score by examining the tombstones (the number will be in red font.) Once you have gotten all the high scores, visit burial chamber 1 on your next playthrough. An event will occur and you will be given an extra Press Turn icon. This essentially increases the number of actions you can perform each turn in battle.
NOTE:Your scores carry over to a new game+ file. You can only get the extra icon in new game+ files, not in your very first playthrough.