It’s like one-time bonuses - “Enrage”, “Barrier”, “Enchant”: a troop gains power to have extra turn after its spell has been cast, and then this ability is spent and therefore removed. For example, Amira deals her true damage - and if she has this “temporary extra turn”, she spends it and does not lose turn. Troops with permanent extra turn, like goblins or Skeleton, do not gain this bonus. And it is considered spent even if after casting 4 or 5 gems were created.
Sorry if it was offered earlier, I didn’t dig forums too deep.