The Upper Partzufim do not receive Light for themselves. The Partzuf Aba is full of the Light of Hochma, but Ima does not wish to receive it, pushes it away, and receives it only under the condition that the souls wish to accept this Light from her. This is why MAN, a request is necessary from the lower.
This principle is widely used in technology and everywhere in our lives, in any changing, living system. Any opening and closing, as well as any regulators, are based on this principle.
It is like a spiritual transistor; there is the Light of Hochma (a plus and a minus), and I send an impulse to the base, starting a flow of electricity. I send a very weak signal, a small desire, but due to this desire, enormous Light begins to flow within the system.
In the same manner, souls wish to receive just a little from Ima and open her up slightly, but she asks for the Light from Aba, and Aba asks from an even higher source, and hence this plea reaches the World of Infinity.
This plea causes enormous Light to be spread through all the worlds in order to bring us, who are at the very bottom, the small drop of Light, which we requested.
This is the principle of our whole life. If the system is integral, then even the smallest change leads to a change in the whole system. The spiritual is a reflection of the harmonious connection between all the elements. If we change even one very small part, we thereby completely change the whole system, causing it to reform in its entirety and reach a new state of balance.
The spiritual system is analogous to systems in our world. If a small impulse is sent towards the entrance of the system, the whole system experiences changes, from beginning to end, and in the opposite direction. It is only after it becomes balanced that the result becomes evident.
Therefore, if even the smallest possible soul somewhere at the very bottom performs a slight correction, it changes all the worlds. The lower this soul is situated, the greater Light it arouses above it. This is like a small valve, which blocks a huge pipe. Just a little bit of effort is necessary to move it, but it opens up the flow of an entire ocean of water.
(From the lesson on the “Preface to the Sulam Commentary of The Zohar” 10.29.09)