Partner Powers are a new element introduced in Pokémon Let's Go, Pikachu! & Let's Go, Eevee!. As you're in battle with your Pokémon, there's a chance that an icon with Pikachu/Eevee and a JoyCon will appear. If you shake the JoyCon, it will activate the Pokémon's Partner Power

The Partner Power comes at random times, but the higher your friendship with your partner Pikachu or Eevee, the higher the likelihood that it will appear. Feeding it Golden Berries will help make it appear as will playing the game with touching paws will also aid in this. If the Partner Pokémon has two happy sparkles after playing, petting or feeding it's the sign that the Partner Power may be triggered next in battle. Each activation will make its points go up, and when it hits 100 then it will activate.

When your partner Pikachu or Eevee is in battle, the Partner Power summons a special powerful attack. This attack increases in strength depending upon your friendship with your Partner.

Name	Type	Cat.	PP	Att.	Acc.	Effect	Pokémon
Pika Papow			20	--	101	The more Pikachu loves its Trainer, the greater the move’s power. It never misses.
Veevee Volley			20	--	101	The more Eevee loves its Trainer, the greater the move’s power. It never misses.

If you don't have your partner in battle, the Partner Power can still activate. In this case, it will provide a special boost to your currently active Pokémon, boosting everyu single one of its stats by 1