Elemental Reactions are triggered by applying certain combinations of elemental effects on a target (enemies, players, or objects).
When an element is applied to a target, it will create an elemental aura that decays over time. Elemental Reactions will occur when a second element is applied onto a target with an existing elemental aura.
Triggering elemental reactions will consume the existing aura by varying amounts depending on the type of reaction triggered and the gauge units of the triggering element. This will shorten the existing aura's duration or completely remove it.
Note that many sources of elemental damage have Internal Cooldown before being able to reapply their element and therefore may not be able to trigger Elemental Reactions on every hit.
Certain more complex mechanics of elemental reactions are explained in-depth in Elemental Gauge Theory, a framework for understanding interaction of elements.
Elemental Reactions have the following effects:
Elemental Reaction damage can be increased by the Elemental Mastery of the character causing the reaction. The in-game interface shows the damage bonus provided by the character's current elemental mastery. This bonus will be applied in different ways, depending on the type of reaction.
Transformative Reactions (unofficial name) include Overloaded, Shattered, Electro-Charged, Superconduct, Swirl, Burning, Bloom, Hyperbloom, and Burgeon.
Amplifying Reactions (unofficial name) include Vaporize and Melt.
Additive Reactions (unofficial name) include Spread and Aggravate.