Fasting (Sawm) carries a two-fold meaning - two seemingly opposing definitions combined into a single word. And sawm, as described in the Qur'an and the hadith, simultaneously fulfills both of these definitions. The primary meaning is to hold back, to refrain from, to abstain - the further meaning is to rise beyond, to move past former limits.
The month of Ramadan is a time in which we hold our bodily compulsions and instincts under strict control, together with our thoughts and our mental states, our moods and desires. We submit ourselves (our nafs) and our accustomed patterns of life to a higher template, one that fosters a regimen of self-restraint within the body and mind and correspondingly seeks an intensification of the life of the spirit. The body is ordered to fast from what it needs, from what is normally allowed to it, from what it desires, from what it craves, from what it may seek on a whim, and from what it habitually seeks - from all that leads to an intensification of the activities of the nafs.
During the interval of daylight, halal (the allowed) transforms into haram (the forbidden) and whatever nourishes the physical body becomes haram. As for the nafs, it undertakes a psychic fast from anger, backbiting, gossip, harshness towards others, from reaching in any manner through any of the senses towards that which is disallowed. All those inclinations which strengthen the nafs, which allow it to inject itself with vigor and attachment into the flux of worldly life are proscribed and denied expression.
The nafs continuously asserts itself through it's ties with the body and according to a complex and ever-shifting world of attraction and desire, knowledge and ignorance that endlessly churns within it. Through its movements and motions, it seeks what it needs and wants and can become, depending on circumstances, complacent or cavalier, disdainful or self-assured, arrogant or fearful, callous or ambitious, lethargic or craving - endlessly acting and reacting within the confines of its limited knowledge. What it does not know it is ignorant of, and what it does not know is infinitely more vast in extent than what it knows. So it's knowledge is forever outweighed by it's ignorance and it's pursuits and actions are indicators of which of these (knowledge or ignorance) it acts upon.
The nafs is in continuous restless motion, but it is a motion that circumambulates around a center of manifold physical and chemical interactions that give rise to need, wants, pleasures, habits, moods, impulsions, compulsions, and desires. The complex system of body and mind are in an incessant state of movement (that ceases only with death), switching continually from one mode to the other, pouring forth a torrent of thoughts and internal impulses that turn the mind's focus endlessly from one locus to another. There is perpetual movement and motion but within tightly constrained boundaries - pivoting around the locus of the nafs and what it seeks.
And so the qur'anic command is issued - "...fast until the night...." (Qur'an 2:187) Fast from what the nafs needs and desires. Let the nafs know that there is a truer aspect of yourself, a center capable of overseeing and stabilizing all the intersecting mental systems of the mind and all the material/chemical/habitual/hormonal systems of the body. Proclaim to it that there is a guardian and owner and ruler over the nafs and over the physical form with which it is integrally co-mingled. Let it know that the form and the stirrings of need and desire within the nafs have to submit to this guardian in seeking their satisfaction. The wants, needs, and desires that spring from the material form must submit to the governance and tutelage of a higher form - to the spiritual form indicated by the hadith that states: "God created Adam in His own form...." (hadith)
This is not the material form driven by chemical interactions but the spiritual substance which is the subtle, essential form of a human being - one that is masked by the ceaseless activity of an unconstrained nafs (nafs al-amarra).
The material form and its impulses (manifested through the nafs) are reigned in during fasting. All the things which give strength, vigor, and life to the body and nafs are terminated - the attachment is reduced, denuded, weakened. We cease to consume and are no longer able to enjoy what feeds our physical form and with that cessation we begin to unhook the clamps which bind us to the most basic goods of this world. We undo the shackles which tie us through our physicality to the world. By penetrating to the very root of our attachment, to the most fundamental layer, to the very seat of our creaturely connection to the world - food, water, sex (the three cardinal symbols of life) we overturn their dominion and arrive at a position where we, for a time, subdue them.
We deny creaturely externals, we let the creaturely demands and impulses remain unanswered - over the course of the days of fasting we let them subside and wane. We let them grow silent so we have a chance to hear what we otherwise would not hear, to perceive what we otherwise could not perceive. We subdue our physical form and when its clamoring grows silent we perhaps become aware of a spiritual form that resides subtly within us.
The vigil of denial and regulation of the physical form and the nafs is maintained until the spirit and mind's ascendancy becomes clear. "Fast until the night...." (Qur'an 2:187) The night approaches and the day's fast ends with the former hierarchy reversed - what was first (physically and psychically generated needs, wants, and desires) comes last and what was last comes first, and with this new ordering of spirit and body in place, the fast is completed. Over the course of the month of Ramadan, as the days merge into the nights, this drama of reversal is repeated and intensified till the person fasting (the person who undertakes the fast with complete sincerity and profound intensity) approaches a state of spiritual readiness.
Until in the watch (the vigil) of the last ten nights of the month of Ramadan, there arrives the possibility of a profound inner remaking, an unfolding of the potential to witness the laylatul qadr. "And what can convey to you what laylatul qadr is? That night is better than a thousand months...." (Qur'an 97:2-3) During the day we break ourselves down, we fast from what sustains our existence - we submit our clay form to be unmade, to be kneaded and worked over - we remove ourselves from our material subsistence and turn to prayer and spiritual subsistence from God - we prepare ourselves to be reshaped. The onset of the darkness of night is representative of pure potential waiting to emerge into existence - waiting for the command and decree which will give it form. "The angels and the spirit (ruh) descend in it, by the command of their Lord with every decree...." (Qur'an 97:4) We turn ourselves into malleable clay awating the shaping command of that night - anticipating the profound and weighty descents that accompany laylatul qadr. "(That night is) Peace till the breaking of the dawn." (Qur'an 97:5)