According to The Book of Zohar everyone should reveal an action below. Therefore Aaron performed an action below, seven days of holiness during which he didn’t come out of the Tent of Meeting, and then he offered sacrifice on the eighth day in order to invoke the action from above by which general unity is revealed. Then all the worlds that the priest (Cohen) awakens receive the blessing, and the priest himself is filled by the wholeness as required.
It is about the need for Aaron’s bestowal (the complete attribute of bestowal) unto the people (the other attributes) just as unto a group, which by gathering, affects the force that is above it since Aaron cannot influence it.
He has to perform an action below in order to unite the people and then they influence the upper force. Then the upper force can influence Aaron even more so that he can unite the nation even more strongly and then they influence the upper force even more. This is a continuous cycle.
It turns out that Aaron and the nation depend on each other. The upper force responds to the request of the nation, to its call, to its prayer (the prayer of many).
Aaron can only be a teacher and raise the prayer of the nation upward, but the nation operates him. It is thanks to the nation that Aaron receives spiritual elevation and all the others ascend through him in their turn.
From KabTV’s “Secrets of the Eternal Book” 12/18/13