From Bulbapedia, the community-driven Pokémon encyclopedia.
In co-op battles, three Trainers team up to battle the opponent. Each Trainer brings a team of three Pokémon but only has one Pokémon out on the field at one time, with each Trainer's Pokémon having their own move gauge. Each Trainer is able to switch between their Pokémon, although switching takes one turn. Once the player completes the "Switch-In Moves!" tutorial in the training area, the player will be able to choose a move during the same turn as switching. Switching has the same effect it has in the core series, in that all the alterations to statistics are reset and passive skills that are triggered at the beginning of the battle, such as Endure and Terrify, are triggered when the Pokémon is switched in.
'''Unity attacks''' are moves unique to co-op battles. These are powerful attacks that are unleashed by all three allied sync pairs combined and target all opponents. To use a unity attack, the team must fill the unity gauge. The unity gauge is filled by using moves in specific ways. When the Pokémon of two different players attack before an opponent's offensive move, a chain is created that will last until the opponent attacks the team. Any Pokémon moves (not Trainer moves) used during a chain will fill the unity gauge equal to the number of move gauge slots required to use the move. Critical hits will fill the unity gauge by an extra +1. If the Pokémon of all the three players manage to attack before the opponent attacks, the team will obtain a unity bonus for a short period of time. When the unity bonus is in effect, the attacks of the opponents won’t stop the chain and moves will do more damage to opponents. The duration of the bonus can be increased by using super effective moves. Once the unity gauge is charged, the team will unleash a unity attack. The move's type is a combination of all allied sync pair's types. Each player can choose one of their sync pairs to partake in the unity attack and to be used on the field afterwards, allowing the player to choose a type that would be advantageous against the opponent. Choosing a different Pokémon from the currently active one is considered a switch, resetting the alterations to statistics. Furthermore, during the attack animation it is possible to choose an action (either a move, a switch or a switch-in move) that will be performed immediately after the attack, before the opponent’s move.