The Press Turn System is relatively new to the gaming world by RPG standards, but takes bits and pieces from previous battle systems to create something much more cerebral, challenging, and fun. The main goal of the PTS is to defeat your opponents by exploiting their weakness while protecting your own. While it seems like old hat, there is much that the system does to differentiate itself.
Flow of Battle
As is hinted at in the name, the Press Turn System is a turn based one (exception to this rule is Devil Summoner), meaning that the player controlled team takes turns attacking and then the enemy gets their chance. One attack is allotted per character deployed in battle. How you use these opportunities is crucial to your chance at winning the fight. By executing an attack (physical, magical, or using an item) one press turn will be used up. If you decide to pass a characters turn then only a half of one press turn is used. As long as there is a half of a turn left, it can be used like a full turn. Thus you can pass the first characters action and still be left with the same number of attack opportunities, a crucial tactic to get skills used in the right order.
To make things more interesting though is the fact that you can lose and gain turns in a few ways. If an attack is blocked by an enemy or if the enemy dodges the attack, you will lose a press turn. So if you only have two moves left and it's blocked, your turn is done. Luckily if you happen to know the enemies weakpoint or get a lucky critical hit, you will gain half a press turn (essentially an extra attack when all is said and done). Thus making enemy exploitation and knowledge of your foes a primary factor in winning the battle. Be aware though that enemies are capable of doing the very same things to you. Therefore if you are unprepared and caught off guard, even a much lower level group of enemies can defeat you quickly in a string of critical strikes.
Differences via Games
Nocturne was the first game to introduce this battle system. It is also one of the most difficult and cruel versions of it as well. The game is known to catch any unprepared or overconfident player unaware and defeat them before they realized what even happened. The number of turns received in battle is based solely on the number of demons you have deployed into battle with you at the time. In addition to the player the most allowed at once is three giving you a maximum total of four turns. Play your cards right and attack up to eight times in a turn. Get caught unprepared or get unlucky and only get two of the four. Strategy and knowledge is key to getting anywhere in the game as every available demon has its strengths and weaknesses. Know your enemy and know your team.
Digital Devil Saga differs from the use of the system seen in Nocturne a few ways. The first and most obvious difference is that you will almost always be using the same characters. Unlike in Nocturne where you could swap out characters to get just the right skills, DDS requires players to equip various skills to the available roster of characters. Equip the wrong skills and you may not be able to damage the enemy at all in battle. Plus since the characters are pretty constant, they are each weak to a different attack type. So there are times you will be forced to have a character that can be exploited in battle with you. Having the necessary skills to counter is the key to success in this game.
Here we have the one exception to the turn based rule of the system. Devil Summoner is an action RPG that uses a modified version of the press turn. Since the game is an action RPG there are no turns in battle as everyone is acting and roaming freely at will. You do still have the ability to call a demon to your side to fight with you but they act of their own free will for the most part. To keep them safe and exploit enemies you must constantly be swapping fighters to get the edge. Also the main character cannot use magic and must use elemental bullets to exploit enemies. Since you can't gain extra turns, exploiting enemies is the method used to stun them to both allow you to capture them or get in free attacks while they can't move.
This version of the press turn system is pretty basic except for a few things. Exploiting a weakness or dealing a critical hit will grant an extra turn for that individual character and will knock the enemy to the ground. Attacking the an enemy while it's already down will not grant an extra turn. Knocking an enemy down serves two purposes. First, a downed enemy can not attack on their next turn because they spend it getting back up. Second, knock all enemies down at once and you get the option to rush in dealing critical damage but returns all enemies that survive immediately stand back up. The other difference of note is that now you can only control your main character in battle. The other three characters you bring with you will act on their own aside from some very basic tactic commands you can use. To allow you more freedom in attacking/defending the main character can switch personas and open up different attacks and spells during battle. The key to this game is having a balanced stock of personas available to switch to. As long as the main character doesn't die you can live to fight again, so defense is just as important as offense.
The system in Persona 4 is very much the same as in Persona 3 with a few differences. The most obvious of which is that you now retain control over all of your characters if you choose. By default your team will act on their own in battle but the option to set their tactic to direct order allows you to play the game much more like a traditional RPG and makes it much easier in setting up specific strategies and tactics than in Persona 3. Another difference to make note of is that it no longer wastes a turn for an enemy to get up from being knocked down meaning that in all but a few instances it is always expected to All-Out Attack your enemy. Attacking an enemy that fell down with another critical hit or a move that exploits its weakness will make the enemy dizzy and skip their next turn.
All of the games to feature the Press Turn System have used it in a slightly different way than the next and all forms have worked well thus far. As more and more SMT games are being released (along with spin-off franchises like Persona, Devil Summoner, and DDS) the system keeps coming along with them. Atlus has crafted a unique system that takes the familiar and tweaks it to make a more challenging and rewarding experience for experienced RPG veterans as well as a unique style that newcomers can come to enjoy just as easily.