Unburden doubles the Pokémon's Speed stat when the Pokémon's held item is lost. If there is no held item to begin with or if a held item is traded for another, Unburden will not be activated. This boost is lost if the Pokémon is switched out or gains a held item.
While this Pokemon is active, it prevents opposing Pokemon from using their Berries. Activation message broadcasts before other abilities regardless of the Pokemon's Speed tiers.