The Gemora cites a braisa: There is a stringency in confirmation over revocation, and in revocation over confirmation. The stringency in confirmation is that if he confirms in his heart, it is confirmed, but he cannot revoke in his heart.

The Ra”n explains: If he confirms in his heart, it is confirmed, but he cannot revoke in his heart. This is seemingly derived from the halacha of remaining silent on the day that he heard, which is regarded as a confirmation. What is the reason? It is because once the day of hearing has passed and he did not revoke the neder, he has revealed what is in his mind, and that is that he wants the neder to be confirmed. It follows that as long as it is in his heart to confirm it, that is sufficient. But the Torah provided for him the entire day in which to revoke it, because until the sun sets, it is not clearly evident that he wants to confirm it. Perhaps after that he will revoke it, since we are going only according to his intention. It follows that whenever he actually confirmed it in his heart, it is confirmed immediately, because it is as if the day of hearing has passed.

Reb Akiva Eiger asks: Perhaps it is only in the case where he remained silent the entire day that it is regarded as a confirmation? For there, it is clearly evident to everyone that he desires to confirm it in his heart. However, in a case, where he is keeping quiet with the intention of confirming it, and no one knows about it, for the day has not passed yet, we would not regard this as a confirmation. Rather, it would be considered “things in the heart,” which are not effective!

The Dvar Avraham (2:14) suggests the following answer: Prior to the conclusion of the day, it is not evident that he wishes to confirm it, for perhaps he desires to confirm it at the last moment of the day. It can also not be said regarding the last moment of the day that we know for certain that he wishes to confirm the vow now, for perhaps he decided previously to confirm it. It emerges that there is no moment in the day that his intent is revealed to the entire world, and therefore, it would be a proof that a confirmation in his heart is indeed regarded as a confirmation.