§ 811.2 Core Rules PDF (officieel)

Abilities and instructions of hidden cards other than the choices listed above function as normal. Example: Stand United is a spell that has Hidden and says “Buff a friendly unit. Buffs give an additional +1 might to friendly units this turn.” If it’s played from Hidden, the first part of its ability must choose a friendly unit at the same battlefield, but the second part of its ability affects all friendly units with buffs, no matter where they are.