Unnerve causes opposing Pokémon to be unable to eat their own held Berries. However, it does not stop against using Natural Gift. When an Unnerve user switches in, the Ability displays a message. If there are entry hazards, then they activate after the message.
Wild Pokémon are 1.2x as likely to answer calls for help when facing a Pokémon with Unnerve. It shares this effect with Intimidate and Pressure.
Unnerve does not prevent opposing Pokémon from eating held Berries when the move Teatime is used.
Outside of battle
Unnerve has no effect outside of battle.