Burst Mode is a special mode that is exclusive to Pokkén Tournament.
In Pokkén Tournament Edit
During burst mode some Pokémon can transform into their Mega Evolution, pokemon that don't like mega evolutions (such as Braixen) just get a special aura around them to show that they are in burst mode. During burst mode, Pokémon are more powerful then their normal form, leading to higher damage potential. Pokémon can also preform burst attacks, extremely powerful attacks that upon triggering, play a unique cutscene that shows the process of the specific Pokémon doing the attack.