When a specific combination of Mystic Arts is used, a Synergy may be triggered by the union, which results in an Arcana being cast. To see what combinations can be used, check the individual Arcana pages for more information.
The union must have the right combination of arts, enough AP and no ailments (Blacked Out, KO'd, Curse, Poison and/or Silence) in order to trigger the Synergy. If the union has the right arts but not enough AP, the individual arts will be used instead. Paralysis and various unit ailments (Burned, Shocked, etc.) do not affect Synergy occurrences.
Although rare, enemy unions may also trigger a Synergy if the right arts combination is used. The magus union inside Base 4, the Jhana Royals, and the Remnant Guards while fighting The Fiery Gates are a few examples of enemies capable of using Arcana via Synergy.