Swaying During Prayer
The practice of making odd and irregular swaying movements during prayer is a unique phenomenon which is virtually exclusive to Judaism. It is generally understood that individuals engage in these movements in an effort to deepen concentration and intensity when praying. The origin of swaying during prayer, often referred to by its Yiddish name, shuckling, is actually quite unclear. Many also sway when studying Torah, as well.
There are, however, a number of theories offered as to the development and the role of these swaying movements. Some authorities suggest that the custom of swaying during study and prayer is meant to recall the trembling that the Jewish people experienced when they received the Torah at Mount Sinai.
According to the Zohar, the practice of swaying is related to the human soul. In Biblical and Talmudic literature, the soul of man is likened to a flame. Just as a flame appears to sway and flicker in an upward motion, so too the Jewish body and soul "flicker" during prayer in an attempt to reach "upward" to higher spiritual levels. It is taught that King David referred to these movements during prayer when he said "Let all my bones exclaim: God, who is like You!" meaning that one should include one's entire body in the prayer experience.
Another theory as to the origins of swaying suggests that it may have been practiced as a means of keeping warm and awake during prayer and study. It is also cited as a remnant of the days of yesteryear when, due to a shortage of books, many people were forced to study together from a single text. The swaying in and out was an exercise which allowed everyone to take turns getting a glimpse of the text. It is also the reason why holy books have historically been printed in very large formats.
It is taught that swaying during prayer is a feature of the Jewish people and that it is a movement which is unique to the Jewish soul. Some sources even rule that swaying during prayer is a "requirement". Swaying may have also been an ancient Jewish posture for study and prayer which was permanently adapted and continues to this very day. Indeed, it is suggested that the origins for swaying during study may have its roots in the Talmud, which would establish swaying as a much older custom than we actually think.
Nevertheless, there have been many great rabbis and other pious individuals over the centuries that never swayed when they prayed. Indeed, there exists alternative approaches as to how one should posture oneself during prayer. This is closely related to the teaching that one should behave as if one were standing before a king when praying to God. In our day and age, when one stands before kings and other world leaders, one stands in a formal and attentive posture. It would be unacceptable to sway while speaking to such people. For this reason some authorities have discouraged any movements during prayer.
While there are those who encourage swaying during prayer and those who frown upon it there also exists somewhat of a compromise approach to the issue. It has been suggested that those who desire to sway should not hesitate to do so during most prayers, especially when reciting the pesukei d'zimra. However, according to this approach, one should avoid doing so during shemoneh esrei. As the shemoneh esrei is the most intimate audience one can ever hope to have with God, one should consider standing still, as during such time an appearance of reverence is in order. This was the practice of Rabbi Moshe Feinstein.
There is a parable attributed to a number of Chassidic masters which teaches that praying is like drowning. When a person is drowning in a river he is going to make all forms of frantic movements in his attempts to extricate himself from the water. So too, when a person prays he is actually saving himself from the dangerous "waters", the distractions of this world, fighting to connect with the spiritual. Even when one does sway during prayer, it is recommended that the motions be of a forward and backward manner, rather than a left to right manner. It is also suggested to only allow one's body to sway during prayer, but not one's head.
Swaying is also reputed to help with one's concentration and expel any improper thoughts during prayer. The halachic consensus is that those who find that swaying during prayer helps their level of concentration should not hesitate to do so. On the other hand, those who are more comfortable standing at attention should do so. One should not suggest for a moment that one's movements and mannerisms during prayer are cause for one's prayers to be given any more or less credence by God. It really doesn't matter what one's posture is during prayer, just as long as one conducts oneself with the best of intentions. One should not imitate the movements of others during prayer, but rather, one should experiment with different movements and choose for oneself that which is most conducive to one's personal relationship with God.
 Rema O.C. 48, Mishna Berura 48:5, Machzor Vitri, Taamei Haminhagim Likutim 90
 Taamei Haminhagim Likutim 93
 Zohar;Pinchas 218b
 Tehillim 35:10, Mishna Berura 95:7
 Zohar;Pinchas 219
 Sefer Hamanhig p.85
 Kuzari Chapter 2
 Eruvin 54a
 Yerushalmi Berachot 1:5, Malmad Hatalmidim 159
 Minhag Yisrael Torah 48:3
 Mishna Berura 48:5, Kaf Hachaim 48:9
 Rivevot Ephraim 3:168:20
 Piskei Teshuvot 48:7
 See Rivevot Ephraim 1:50:11
 Tzror Hamor;Yitro, cited in Piskei Teshuvot 48:7
 Sefer Doresh Tov;Yetzer Hara, cited in Piskei Teshuvot 48:7
 Aruch Hashulchan O.C. 48:3
 Pri Megadim 48:4
 Minhag Yisrael Torah 48:3