In addition to serving as a controller for Pokémon Let’s Go Pikachu and Let’s Go Eevee, as well as a functioning Pokémon GO Plus, the Poké Ball Plus allows you to send a Pokémon into it from Pokémon Let’s Go. Doing this will grant you some in-game rewards and benefits. If there’s a Pokémon in your Poké Ball Plus and it’s running in Pokémon GO Plus mode, it will automatically spin nearby PokéStops for you. If you don’t have a Pokémon inside it, you’ll instead receive notifications via vibrations.
Source: topics.nintendo.co.jp
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