Final Fantasy XII
Although it does infact share the same name as a term found in the Highlander Universe, in Final Fantasy XII the term Quickening refers to something vastly different.
Unlike previous incarnations of the 'limit break' system, Final Fantasy XII decided to completely break the mould and have its system unrelated to a player's actions. Instead, the ability to perform a Quickening relates to how many Mist Charges they have, a metric which is decided by how much MP the character has.
How Many Quickenings Can A Character Have?
Final Fantasy XII was both criticised and praised for the customisation it allowed players, with every character able to learn everything. This isn't true for Quickenings though, as they are all completely independent. It also doesn't matter which Quickening is purchased on the licence board as it just continues with the character's natural progression.
Each character has a possible three Quickenings they can learn and they can be learnt in any order irrespective of the other characters. The only hinderance is that once someone has used a Quickening licence, that space cannot be used by anyone else.
Once a Quickening is learnt, it gives the character a larger MP pool, and a Mist Charge. If the character has unlocked all three Mist Charge bars, but they only have enough MP to fill up one, then they can only perform the lowest order of Quickening.
There's No 'I' In Team
Although Quickenings can be powerful on their own (especially the ones that require three Mist Charges), they are much more deadly when combined with Quickenings performed by other characters. If the other characters in the party also have Mist Charges available, during the performance of a Quickening the player has the option to chain with another Quickening.
To chain, the player is given a list of options, which are created from a combination of the Quickenings the other characters can perform. If they have a button next to their name, they are able to chain. Pressing the associated button means they will then perform their Quickening next. However, if no buttons appear, or a suitable Quickening isn't there, pressing R2 will shuffle the options. There is only a limited amount of time though, so time is of the essence.
As well as having the ability to chain Quickening attacks together, Final Fantasy XII also added rewards for successfully reaching certain milestones. There were eight different Concurrences that could be performed, but which one appeared was dependent on the number of Quickenings performed and how many Mist Charges were used.
Final Fantasy XII: Revenant Wings
Quickenings returned in Revenant Wings, but as with other aspects of the game, they were also simplified. Characters no longer had multiple Quickenings and they could no longer be chained together. This meant that they were much more similar to the Limit Breaks of old.
Quickenings were now rewarded to characters after a certain Tier III esper had been defeated, for example Vaan learns Pyroclasm after the group defeat Belias and Basch learns Apocalypse Shield after the group defeat Zalera.
The Mist Gauge Was Too Vital To Be Mist Out
One of the aspects of the Quickening that was retained for Revenant Wings was the Mist Gauge. Although its role in Final Fantasy XII was more important due to each character having three Quickenings, in Revenant Wings it was still fundamental aspect of being able to perform a Quickening.
It was possible to charge the Mist Gauge by performing attacks in combat, or receiving damage. This made it much more like a traditional Limit Break gauge.