The PA increases all damage when at an elemental advantage. Say Double turns to Fire element and the enemy is Air. Without the PA, Double gets the normal elemental advantage of 20%. A 500hp punch becomes a 600hp punch. Double's PA enhances this gain, so when the PA is maxed out to 100%, that 500hp punch becomes 1000hp. Essentially living up to the "Double" name by now being able to do double damage with any kind of attack.