In his commentary to Genesis 12:6, Nachmanides writes:
“Know, that any ‘decree of the watchers’ (i.e. prophecy, see Daniel 4:14) when it emerges from the force of a representative act (poel dimyon) the decree must absolutely be fulfilled. For this reason, prophets perform prophetic actions such as Jeremiah’s command to Baruch to tie a stone to the scroll and to throw it in the river and to say ‘so will Bavel sink.’ (Jeremiah 51:63-64), so too when Elisha placed his arm on the bow… (Kings 13:17).
There are numerous examples of prophets performing actions that represent the content of their prophecy. Nachmanides explains that such actions serve to ratify those prophecies and make them absolutely certain to be fulfilled. (The implications of this idea for prophecies that do not involve such actions is discussed at length by the Rabbenu Nissim in Derashot HaRa”N, derasha 2)
Think of the sound of a synagogue full of aravot hitting the floor. The sound is almost identical to that of a hard rainfall. The aravot — willows of the brook — which require abundant water hitting the ground like rain and making the sound of rain. We perform this representative action and say “Kol mevaser!” “The sound heralds!” After many paragraphs of pleading with G-d to save us “Hosha Na!” we take our prayers to another level. Just as a prophet ratifies and secures the positive outcome of his prophecy through representative action, we make our prayers more likely to be fulfilled by performing an action that represents those prayers. It is no longer merely a request. We have done a representative action. In the words of the Nachmanides, “the decree must absolutely be fulfilled.”
To put it another way: When we hit beat the aravot on the floor we behave like prophets. Minhag neviim — a custom of the prophets — indeed!
Others have suggested that חיבוט ערבות represents a prayer for rain, the sound of beatingערבות evoking the sounds and sights of a rainstorm.[6] This seems plausible, as the Talmud and Midrash indicate that arba minim and other practices of Sukkot are in part connected to our prayers for rain, which begin at this time of year. However, once again this explanation fails to shed light on why the custom was initiated specifically by the prophets of שיבת ציון, or how we can possibly reconcile the seemingly self-contradictory, dual status of מנהג נביאים and זכר למקדש.