The descent of a soul into this world transports it from a purely spiritual realm into a gross physical one. To facilitate this descent, the soul spends many months within the womb. This serves as a transitional stage between the physical and spiritual worlds. In the womb, “a lamp is lit over its [the embryo’s] head ... and it is taught the entire Torah”, an experience associated with the upper world. At the same time, the soul’s existence in a physical body brings it to experience the lower world as well for, “it eats what its mother eats and drinks what its mother drinks.”
On Shabbat, every Jew receives a Neshama Yetera – additional soul. The need for an ‘intermediary stage’ also applies to this Neshama Yetera. Two angels escort this soul down from its lofty, divine abode. Another two angels that normally dwell with the person ascend upward to meet the additional soul and to carry it back to the person. The upper and lower angels thus meet halfway, creating a bridge that enables the Neshama Yetera to descend. In the Shalom Alechem poem it is the upper angels whom we greet, for the lower angels are with us constantly, throughout the week.
Why then do we bid the angels farewell at the conclusion of the poem? It is taught in the Talmud that a bride remains in the possession of her father until he hands her to the groom. In a case where the father sends emissaries to take his daughter to the groom and the groom, in turn, sends emissaries to receive the bride, as long as the father’s emissaries remain with the bride she is still in the possession of her father. Only when the father’s emissaries leave her on her own with the groom’s emissaries is the bride considered in the groom’s possession.
The same principle can also be applied to the angel’s exchange of the Neshama Yetera. As long as the first two angels from the upper realm still accompany the Neshama Yetera, it remains connected to the upper domain and is unable to enter our soul. By bidding farewell to these angels once they have delivered the Neshama Yetera to our lower angels, we ensure that the Neshama Yetera is in our possession.
Based on Shem MiShmuel, Bereishit p.236