The skill is listed in the special tab. It's always active after it unlocks and basically adds Aura's level to that of her allies. You can see the effects when you check your allies' status. The Pokémon battle itself serves to showcase how starshine means your allies can suddenly handle enemies they used to struggle against.
An early +1 experience/day can give your allies an extra level for the festival, this means +4 damage on a 4 power attack and -2 damage from enemy attacks that have 2 mitigation(which is most of them). For most of the festival battles, it means needing 1-2 fewer hits to kill an enemy and enemies maybe needing 1 more hit to kill your ally. Faster kills and less damage taken means less healing necessary, saving you gold/perika.
Incidentally, Charlotte doesn't gain experience while in prison.