Initiating a Combo Attack. A combo attack can be initiated whenever a player character makes an attack on their turn, before the die is rolled but after the attack is declared. The character who makes that attack is instigating the combo, so they are referred to as the instigator. Participating in a Combo Attack. Other player characters who can see the instigator can each use their reaction to enhance the attack by performing a Combo action. A character who assists in this way is referred to as a collaborator. In the case of a combo attack with more than one collaborator, the collaborators take their reactions in initiative order. Once any collaborators have taken their reactions, the instigator makes their attack roll to finish the combo attack. Limitations. Participating in a combo attack is an exhausting endeavour. Once a player character is an instigator or a collaborator in a combo attack, they can’t instigate or collaborate in another combo attack until they finish a long rest. Combo attacks also require participants to be familiar with each other. Only characters who have spent at least one week adventuring together, over which time they observe one another’s fighting style and movements, have the integral knowledge required to perform combo attacks together.
Source: Ryoko’s Guide to the Yokai Realms