From publisher blurb:
Teamwork is small, and easily digestible ruleset designed to allow two individual players to form a close connection with abilities to complement each other. Players can choose to form a Team. Teams are either treated as a 'Partnership', or as a 'Rivalry'. As the team grows, they gain synergies.
Synergies allow the two players to share each other's abilities, boost and benefit one another, and even perform new actions they normally wouldn't be able to do. Group Synergies benefit both members of the team, while Individual Synergies benefit only your teammate. It has been carefully tested so that it is fun, functional, and will not overpower the challenge rating system, but merely provide small benefits in specific situations, and give teammates more options in roleplay and combat.