Rav Yehuda quotes Rav as teaching that even after a person searches his house to remove the hametz, he needs to be mevatel – to nullify – the hametz, as well.
Rava said: The reason for the requirement to render leaven null and void is based on a decree lest he find a fine cake [geluska] among the leaven that he did not destroy and his thoughts are upon it. Due to its significance, he will hesitate before removing it and will be in violation of the prohibition against owning leaven.
According to some manuscripts, Rav Yehuda’s original ruling obligates a person to nullify the hametz be-libo – in his heart. Based on this reading the Ran and the Ramban explain that the main issue here is that a clear mental decision should be made that the hametz is valueless to the person as Pesah begins. The Ritva indicates that even if there is no obligation to make a statement out loud, ideally the person should say the words of nullification. Others argue that the Gemara is trying to emphasize that someone who says the formula of nullification should make sure that he means it in his heart, as well.
The Ran points out that on a Biblical level, searching for hametz and destroying it is enough preparation for the holiday, and even if some hametz is left, there would be no transgression. Rava’s explanation that we fear finding hametz that had been missed accounts for the Rabbinic concern about places where hametz is not ordinarily brought, places where there is no obligation to search at all. The Ramban is quoted as saying that the nullification discussed here does not mean to declare the hametz hefker – ownerless – rather it is a statement that for the upcoming holiday hametz is considered something that is repulsive to him.
According to Jewish tradition, we recite the formula for nullifying the hametz that appears printed in haggadot. This Aramaic formula is derived from the Geonim, who translated it from a Hebrew version that appears in the Jerusalem Talmud. Given what we have learned, it is important to make sure that the text is not simply read, but its meaning is understood and accepted..