There is a Minhag (custom) established by the prophets to take Arabot and bang them on the ground on Hoshana Rabba. The custom is to take five Arabot, tie them together, and bang them five times on the ground, without reciting a Beracha. (Since this practice is only a Minhag, it does not warrant the recitation of a Beracha. This Halacha is known by the expression, "Habit Habit Ve’lo Berich.")
The reason for this custom relates to the comment of the Midrash that the leaves of the Araba symbolize the lips. Specifically, the Arabot represent the lips of the Satan, and we bang them on the ground in order to silence the Satan so he does not prosecute against us. Hoshana Rabba is the day when our final judgment is sealed, and we therefore seek to silence the Satan in an effort to guarantee a favorable judgment. Furthermore, the Arizal (Rav Yishak Luria of Safed, 1534-1572) instructed that one should have in mind while banging the Arabot that the five beatings should correspond to the five letters in the Hebrew alphabet that appear only at the end of words (the final Kaf, the final Mem, the final Nun, the final Feh, and the final Sadi). These letters signify the Geburot – the harsh judgments in the world – and we bang the Arabot on the ground to express our desire to eliminate these judgments. The custom of the Arabot also commemorates the practice that was observed in the Bet Hamikdash to walk around the altar with an Araba on each day of Succot, and seven times on Hoshana Rabba. We commemorate this ritual by using Arabot on the day of Hoshana Rabba.
The Arabot should not be held together with the Lulab, and one should not use for this Misva the Arabot that had been used with the Lulab. If, however, no other Arabot are available, then one may use the Arabot from the Lulab. It is customary to recite an introductory "Le’shem Yihud" prayer before banging the Arabot. In some versions, the text mistakenly refers to this custom with the term "Yesod Nebi’im"; the text should actually read, "Minhag Nebi’im."
The Rama Mi’Pano (Rav Menahem Azarya de Fano, Italy, 1548-1620) ruled that one should not bind the five Arabot together, but others, including the Hida (Rav Haim Yosef David Azulai, 1724-1807), in Birkeh Yosef, disputed this ruling, and the accepted practice is to tie the five branches together with a rubber band.
The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) writes that one should bang the Arabot directly on the ground, and not on stone or pavement. There are mystical concepts associated with beating the Arabot directly on the ground, and this is indeed the proper practice.
All five branches should be at least three Tefahim – or 10-11 inches – long, just the like the Arabot in the Lulab. The Arabot must also be fresh, and not withered. Strictly speaking, it suffices for only part of the branch to be fresh, but as noted by Rav Hai Gaon (969-1038), it is proper to try to ensure that the entire branch is fresh, especially given the availability of fresh Arabot.
The Rama (Rabbi Moshe Isserles of Cracow, 1525-1572) writes that one should first wave the Arabot before banging them on the ground. Many people do not follow this custom, but it is advisable to wave the Arabot to satisfy this view.
The Ben Ish Hai cites the ruling of the Bikureh Yaakob that one should forcefully bang the Arabot so that the leaves will fall off the branches. However, the Ben Ish Hai disputes this view, noting that the Araba branches must remain suitable for the Misva throughout the banging, and therefore the leaves should remain on the branches.
When necessary, one may use Arabot that had already been used for this Misva, as long as the majority of the leaves are still on the branches.
Women are exempt from this Misva, as it falls under the category of "Misva Aseh She’ha’zman Gerama" (Misvot that apply only at specific times, from which women are generally exempt).
It is customary in Jerusalem to return inside to the synagogue after banging the Araba, open the Hechal, and recite the entire text of Nishmat, until the concluding Beracha of "Baruch Mehulal Be’tishbahot" (without Hashem’s Name). The congregation then says a special declaration expressing their intent to recite Nishmat the next year on Hoshana Rabba, as well.
(Taken from Yalkut Yosef, starting on p. 347; listen to audio recording for precise citation)