This was found on 2ch and was supposedly written by a SE employee.

http://hissi.org/read.php/ff/20140928/WkMrcHBtY3Ew.html
Here's a translation:

- Hold Combo~ The AI reads the situation and performs the combo, but you can freely choose which weapon comes at the beginning and the end of the combo. The final hit becomes a strong attack.
When there are no enemies and (or?) you hold the attack button the weapon you used last will be set as the first. Depending on the beginning (first) weapon the evasion action and things like that change.
The attacking motion is slow to make it easier on the player to check the weapon he is using and decide whether he wants to use that one or not in the end.
Depending on which weapon you use for the final hit you can do stuff like launching enemies in the air or tie in other actions (like using items or magic).
But, if you get greedy and attack too much you will die quickly. You will die easily to AoE attacks and the like.
In other words, 15 has been changed from high speed action to that which favors strategy; however, since it emphasis is placed on each attack you make, the combat still feels refreshing and it is very easy to do with just one button.
Also, currently a weapon change icon has been added so you can know which weapon will be used. Whether it adds to the speed is being investigated (?).

- Regular attacks, evasion ~ This can be performed by pressing once. You can tap it as well, but your weapon will not change. You can dodge to whichever place you want.

- Support actions ~ Because you can only control the main character, it's hard to get an actual feel for your party's leveling up, but support actions have been added to have the player get a feel for this.
As their level grows, the types of support attacks increase and you can set this freely making for a variety of actions.
For example, you get a feel like you're controlling/moving both Noctis and Prompto at the same time (?) and you can use Prompto's attack to shoot enemies in the air.
Also, there is another link system in which your party covers you when you make a mistake.

 Here's a good very describing how the phantom swords work also:
https://www.youtube.com/watch?v=viTHUs_hO4c&list=WL&index=1