Each year, on the seventh day of Sukkot Jews engage in the following ritual. First, we walk around the synagogue circling the Torah scroll seven times while holding the four species; i.e., palm branch, citron fruit, myrtle, and willow (see Leviticus 23:40). After these seven revolutions we put down the four species and each member of the congregation picks up five aravot — willow branches. After reciting additional liturgy, everyone beats the willow branches on the floor five times. The entire liturgy leading up to this ritual act focuses on prayers to G-d for rain in advance of the coming winter season. The central phrase of this lengthy liturgy is Hosha-na — Please save. It is after this phrase, repeated countless times in the service, that the entire ritual is named. Hoshanot

At the very end of Hoshanot just after beating the ground with the five aravot, it is customary to say a yehi ratzon — a specific and direct request to G-d. In this paragraph, we use an unusual — to my knowledge, unique — phrase. We refer to the practice of beating the aravot as minhag neviim — a custom of the prophets. The simple understanding of this phrase is that the custom in question was instituted by the neviim — the prophets. Why does it matter that the custom was instituted by neviim? Why mention this historical fact in a yehi ratzon asking G-d to answer our prayers? Why does the prayer stress that it is a minhag neviim by mentioning it twice?

Just prior to beating the aravot we recite a second unusual phrase. Kol mevaser, mevaser ve’omer — “The sound heralds, heralds and states”. What does this mean?

I believe that these two unusual phrases explain each other and contain the key to understanding what hoshanot are all about.

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!