The Quickening glitch is a glitch in Final Fantasy Tactics, which allows player characters to act infinitely. To set up, player at least needs three characters, all equipped with the Time Mage's reaction ability, "Critical: Quick." All of them preferably have nearly 100 Bravery, high Faith scores, or by equipping the Rod of Faith to gain auto-Faith status. Two units have Arithmeticks equipped and have already learned any Black Magick attack and the Mystic Arts Disbelief. The other characters should concentrate on direct attack because any action that requires charging is not going to work on this trick.
The problem here is how to make all allies enter the HP Critical status. Players can follow the video sample to use some stable area attack, which ignores evasion, to adjust the units' maximum HP to enter the critical status. The other, slightly cumbersome way, to accomplish this is by equipping Chemist's support ability, "Reequip." All characters should wear no armor which gives HP bonus when entering the battle. In the first turn, the player should have them re-equip the best armor which boost their maximum HP to create a critical team. In any case, the player must avoid any shield or accessory that gives a magic evasion bonus.
Once critical, the player should calculate Disbelief to target all of the allies. (Whether the enemies are hit by Disbelief or not does not matter.) Lastly, the player calculates any Black Magick attack such as Fire, Thunder or Blizzard, which have the basic hit-rate of 100% to all allies. Since the caster is under the Atheist status by Disbelief, the damage will be "zero," but the game still regards this as HP loss, and will effectively trigger "Critical: Quick". Self-injury does not trigger "Critical: Quick," so there must be at least two units to cast the offensive spell in order to sustain the Quickening cycle. While the casters keep calculating the Black Magick to trigger the Quick status, the other allies can act infinitely as player wishes, although one must be careful to avoid the enemies' counterattack.