It is known that on Shabbat one receives an "additional soul". To understand the concept of the additional Shabbat soul, we must first discuss the three dimensions of the soul. These are the Neshama (from the Hebrew word "neshama", meaning "breath"), the Ruach (literally, "wind"), and the Nefesh (from the Hebrew word "nafash", meaning "rested"). Because of its lofty origin, the Neshama is not completely connected to the body…
The Neshama rests upon the brain; the Ruach dwells in the heart; the Nefesh is connected to the liver. The Nefesh and the Ruach, which are responsible for human emotions, are directly connected with the body. However, because of its lofty origin, the Neshama is not completely connected to the body; body and Neshama are two separate entities dwelling together.
At the moment of death, the Neshama leaves the body. The Nefesh remains, and the Ruach fluctuates back and forth, returning annually on the anniversary of the death to dwell in the body for that day.
The Nefesh is bound to the body's material needs and tendencies. It is an animating force, the motivation behind our basic life forces such as eating and drinking. The Neshama deals exclusively with matters of the spirit. It is an intellectual soul longing to serve the Creator; its goal is to teach man how to make his life a constant fulfillment of the will of G‑d.
The Ruach is in constant movement between the Nefesh and the Neshama. On weekdays, the Ruach attaches itself to the Nefesh; on Shabbat, in which the Ruach is not involved in any mundane activity, it separates from the Nefesh and attaches itself to the Neshama. In addition, for the duration of Shabbat, each Jew receives a "Neshama yetera", an "additional soul". When Shabbat departs, the Ruach returns to the Nefesh, and the additional soul returns to Heaven. One is able to soar to new heights, boosted by an increased understanding of G‑d's Torah…
There are two aspects of Shabbat observance: outwardly, it is a day of rest, but inwardly, it is a time of soul-union with our Maker; in the same way the additional soul has an inner and outer purpose. This outer purpose is, as Rashi explains, an expanded heart, or in other words a sharpening of our sense perceptions comparable to the effect of mind-altering drugs which heighten the ability to see colors, taste food, appreciate sound, and the like. This outer purpose helps us fulfill the commandment of delighting in Shabbat.
The inner purpose of the additional soul is to help us focus our entire mind on the Almighty. As a result, our ability to feel His Presence is intensified, particularly at the moment of prayers. This special focusing power of the mind also results in a higher level of understanding, which greatly enhances the pleasure of learning Torah on Shabbat.
Shabbat provides a unique opportunity to study the Torah for the sake of giving pleasure to the Beloved, without thinking of the reward we will gain. At the time one is absorbed in his study, his/her body ceases to call attention to its material needs. As a consequence, one is able to soar to new heights, boosted by an increased understanding of G‑d's Torah.
On Shabbat the gates of the light of the Torah are opened. The mysteries of the Torah that you begin to grasp increase your faith in the Divine Wisdom and intensify the delight that floods you at the time of prayer.
Whereas the Neshama that a person has during the week enables him to understand the outer aspects of the Torah, the additional soul enables him to understand its mysteries. "Let Him kiss me with the kisses of His mouth", says the first verse of the Song of Songs, which Rashi interprets as:
Let Him be intimate with me again and transmit to me the innermost Secrets of His Torah directly, "mouth to mouth", as He did at Sinai, when He revealed Himself to us "face-to-face."
Thus, it is traditional to read the Song of Songs at the onset of Shabbat to remind us that on this day our wish is fulfilled through the presence of the additional soul.
Hence, the outer purpose of the additional soul - the sharpening of the senses - is to feel the Shabbat delight.
[From "Living the Kabbalah: A Guide to the Sabbath and Festivals in the Teachings of Rabbi Rafael Moshe Luria".]