Currently, "Tag Team" modifier is an aura which causes the affected units slow when being attacked (maybe also grants the attacker attack damage when attacking). Except for this, do nothing. But it shows "Movement speed slowed by %dMODIFIER_PROPERTY_MOVESPEED_BONUS_PERCENTAGE%%% and taking extra physical damage.", which is what "Tag Team Slow" modifier does. "Tag Team Slow" modifier doesn't have its modifier name.