This concept began in Final Fantasy XIII to subtly encourage the use of multiple Paradigms during battles and has carried over into Final Fantasy XIII-2. It isn't mentioned explicitly anywhere in the game so it's a bonus to those observant enough to notice it.
With the ATB Refresh, when you shift Paradigms, the ATB Gauge will refill itself completely. This means that you don't have to wait for the ATB Gauge to refill every time you shift Paradigms. The first ATB Refresh will always occur after the first Paradigm Shift.
After that, you can obtain refreshes if you fulfill at one of two conditions:
- You've previously fully recharged the ATB Gauge
- 12 seconds have passed since the last ATB Refresh
When you don't need a full deck of six Paradigms, particularly in regular (non-boss) battles, shifting between duplicate Paradigms (like two Relentless Assaults) will enable you to attack faster without you having to wait as long in between turns.
Once you've gained more ATB Segments and you're comfortable with the timing of your party's actions, you can reliably use the ATB Refresh approximately once every other turn. This will enable you to speed up battles, get higher scores, and be more efficient when farming monsters for specific spoils, gil, or CP Rewards.
When you shift Paradigms, you have the ability to Buffer moves. This allows you to apply the bonuses of other roles to another ability by shifting to another role right after your character begins to execute that ability. For this to work correctly, you have to make sure the character has actually begun to use the move and it's shown visibly on-screen.
The most obvious example is a shift from Ravager to Commando after using Serah's full ATB ability, Ultima Arrow. While it's not normally used as a main method to deal damage, its power can raised quite a bit by Buffering it into a shift of Commando.
While Buffering can be useful, know that by Buffering, you're giving up the originally intended role bonus for the original ability. For example, a Synergist is casting status enhancements and sees that the enemy is about to use a strong attack and therefore switches to the Sentinel. By doing this, the Synergist Buffers their last spell by shifting to the Sentinel role. Therefore, the bonus duration that would have been applied to the last spell that the Synergist would have cast is lost and the Synergist role bonus will not be applied in exchange for the defensive strength of the Sentinel.