Partner Powers are one of the newest elements added to the Mainstream Pokemon Games, being included in none other than Pokemon: Let's Go Eevee. Partner Powers are seen when in battle, randomly appearing in an icon of a Eevee and then an image of a JoyCon. By simply shaking the JoyCon you are using to play, your Pokemon's Partner Power will activate. The higher level of friendship you have with your Eevee, the greater the chance of a Partner Power appearing when in battle.
If the Partner Power is activated while your Eevee is in battle, it will ultimately do a very strong attack. The greater the friendship you have with Eevee, the stronger the attack will be. If you have a Pokemon in battle other than your Eevee when the Partner Power Icon appears and is activated, a special boost to one of the Stats of said Pokemon will occur.