The Rishonim disagree about fundamental question: Is ḥametz that was batel be-shishim before Pesaḥ ḥozer ve-ne’or (“reawakened,” i.e., its nullification is reversed) when Pesaḥ arrives, and since on Pesaḥ ḥametz in not batel even in less than one part per thousand, it renders the entire mixture forbidden? Or does its bitul before Pesaḥ mean that it cannot be ḥozer ve-ne’or? For example, a crumb of ḥametz that falls into a large dish of cooked meat before Pesaḥ is obviously batel, and this food may even be consumed after midday on the 14th of Nisan. The question is whether it is still permissible to eat it after Pesaḥ begins.
Some poskim rule that if ḥametz was batel be-shishim before Pesaḥ, it is considered completely eliminated, and it cannot be ḥozer ve-ne’or on Pesaḥ. Therefore, the entire mixture is permitted for consumption (Rosh, Smag, Tur, and others). Other poskim say that the annulment that takes place before Pesaḥ is not effective; as soon as Pesaḥ begins the ḥametz reawakens, and the entire mixture is rendered forbidden (Rambam, Rashba).
This question has important implications for the status of matzot. Occasionally, some water drips on a mound of wheat grains, causing a few of the grains to leaven. It is very difficult to find these grains and remove them from the pile, but it is clear that the kosher wheat grains that did not become ḥametz outnumber the leavened grains by more than sixty-to-one. According to the view that ḥametz is ḥozer ve-ne’or, if all the wheat is milled together and matzot are baked from its flour, it will be forbidden to eat them on Pesaḥ, because the drop of ḥametz in it renders all of the matzot forbidden. It is therefore important to make sure that there is not even a single ḥametz grain in the wheat from which matza is made. But according to the opinion that ḥametz nullified before Pesaḥ is not ḥozer ve-ne’or, the matzot are kosher for Pesaḥ. There is no need to check the wheat kernels one by one in order to remove the leavened ones, because they were already batel be-shishim before Pesaḥ.