Our Sages state, “If one wants to invalidate his vows for the whole year, he should stand on Rosh Ha-shana and say, ‘All vows that I make in the future are hereby void’” (Nedarim 23b). This is called “issuing a disclaimer” (“mesirat moda’a”) in halakhic parlance. When we recite Kol Nidrei at the start of Yom Kippur, not only do we nullify past vows, we also issue a disclaimer when we say, “From this Yom Kippur until next Yom Kippur.” Even so, the custom is to issue a disclaimer on Erev Rosh Ha-shana as well, while annulling vows, because those who are enthusiastic perform a mitzva at the earliest opportunity (“zerizim makdimim le-mitzvot”). The earlier declaration is also helpful to one who is late for Kol Nidrei or does not understand that Kol Nidrei involves issuing a disclaimer.
According to most poskim, the disclaimer is effective as long as the person making the vow has forgotten that he made such a declaration. However, if he makes a vow while aware of the disclaimer, his vow takes effect, for by making the vow, he implicitly annuls his disclaimer (SA YD 211:2).
In practice, one who made a vow does not rely on the disclaimer to annul it. Rather, if he does not want to fulfill the vow, he must go to three men to annul it. There are two reasons for this: First, some maintain that a disclaimer is effective only in the unusual case where the person remembers it immediately (tokh kedei dibur) following the vow and intends for the declaration to nullify the vow. However, if the person making the vow did not recall the disclaimer immediately, the vow remains valid. In practice, we take their position into account (SA YD 211:2). Second, even though most poskim say that the disclaimer is effective and nullifies the vow, they still require the person to have the vow annulled before three men. This is because there are cases where the vow takes effect according to the majority opinion, such as if he makes it while remembering that he had issued the disclaimer, and the Sages were concerned that the seriousness of such vows would be undermined if most vows are automatically annulled by the disclaimer (Mahari Weil; Rema YD 211:1). It should also be noted that all agree that the disclaimer is not effective in the case of a vow that one person makes to another, because such a vow is not solely under his control (SA 211:4).
The disclaimer effectively ensures that good practices that one carried out three times are not considered like vows. Without the disclaimer, unless someone explicitly stipulates that he is not making a vow (e.g., he says “bli neder”), doing something three times consecutively is considered a vow. Likewise, a disclaimer is effective for one who resolves to give charity or do another mitzva and does not explicitly state that it does not have the force of a vow. Without the disclaimer, since he did not say “bli neder,” his resolution is considered a vow (Da’at Torah YD 211:2).
Technically, if one issues a disclaimer out loud, it is effective even if he is alone, and his vows are thus nullified. Nevertheless, it is better to issue the disclaimer in the presence of three men. In fact, this is how we nullify vows on Erev Rosh Ha-shana (Ritva, Nedarim 23b; Kol Nidrei 81:10).
If a woman attends Kol Nidrei and understands the meaning of the disclaimer, her vows are annulled with its recitation. If she does not attend, it is proper for her to recite the disclaimer to herself. Thinking it is not enough; she must actually verbalize it.