Manafont is a secret technique that allows a Black Mage to awaken the magical power sleeping within their body and cast magic spells without expending MP. While using this ability, magic you cast cannot be interrupted by an enemy attack.
When active, the MP cost of all magic is reduced to zero, allowing spells to be freely cast as long as recast timers permit.
When active, the casting of any type of spell becomes uninterruptible by melee damage, an effect that is normally limited only to Songs. Movement, along with Stun, Sleep, Silence, and other such debilitating status effects that halt spellcasting may still cause interruptions.
Often used as emergency casting of healing magic when all mages are spent on MP.
Sometimes used as a means of ensuring the success of a Warp or Escape spell.
Used along side Mana Wall to mass kill mobs in Abyssea. Refer to BLM's FC in talk for more.
Since Manafont is a one hour special, some people prefer not to make a macro for it to prevent unintentional activation.
Some people prefer to put one hour job abilities on a trigger macro. This means they use the macro syntax /ja "Manafont" <stpc>, which makes the macro user have to select a player character to use the job ability on, increasing the safety of the macro.