The taking of an aravah, a willow in the Bet Hamikdash is a halachah l'Moshe miSinai (a halachah which Moses was given at Sinai). The taking of this willow on each day of the Festival was independent of the mitzvah of the four species, for this willow was used for the hakafot around the altar. Unlike other examples of halachah l'Moshe miSinai, the taking of the willow applied only to the service in the Bet Hamikdash itself. The Prophets Chaggai, Zecharyah, and Malachi, however, instituted the custom of taking the willow on Hoshana Rabbah, along with the prayers for water and Divine blessing throughout the year, even outside the boundaries of the Bet Hamikdash.
Five willow branches are taken, like those used as part of the four species, and are bound together. Although we are not as strict regarding the fitness of this willow as we are regarding the willows bound together with the lulav, we attempt to find the most beautiful ones available in fulfillment of the verse (Exodus 15:2): This is my G-d and I shall exalt Him. After the conclusion of the hakafot, when the etrog and lulav have been put aside, we recite the piyutim from Ta'aneh Emunim through Kol Mevaser. The bound willows are then beaten on the ground five times and put away in a place where they will not be stepped on since they have a certain measure of sanctity and should not be discarded haphazardly.
The custom of beating the willows on the ground has great mystical meaning known only to the very wise who merit the knowledge of these secrets. Those who have no understanding of the esoteric should have the intent of following the custom of the Prophets and Sages of all generations. Their reward for emulating these actions is that G-d will regard them as if they had indeed acted with the proper, profound intentions.
