The Pokémon's heart fills with memories, causing its Mask to shine and one of the Pokémon's stats to be boosted.

Upon Terastallizing, Ogerpon's Ability will change to Embody Aspect, and one of its stats is raised, depending on the mask.
Teal Mask: Speed
Wellspring Mask: Sp. Def
Hearthflame Mask: Attack
Cornerstone Mask: Defense