Matza Shemura, What is it? Why do we need it?
When Gd commands us that during the days of Pesach we are not to eat Chamets, it is understood that all necessary precautions must be taken to ensure that what we eat is in fact not Chamets. Just as we are vigilant with other Torah guidelines that govern what is Kosher and what is not, this requires no less care and attention to detail.
Consequently, it is unnecessary and redundant for the Torah to explicitly state. "You must guard the Matzos" (Shemos 12:17). The Torah must be conveying an additional message.
Our Sages explain that the Torah requires that in addition to ensuring it is not Chamets, Matza must be made for the express purpose of fulfilling the obligation, the Mitzvah of eating Matza.
Matza made perfectly in all respects but not for this purpose, can not be used to fulfil one's obligation to "eat Matza on this night". Eating this Matza at the Seder is a special Mitzva that has its own special Beracha: Asher KiDeShanu BeMitzVoSav VeTziVaNu Al AchiLas Matza.
The Gemara explains that we may eat unleavened bread during Pesach even if prepared by a non-Jew since it is not Chamets. However, it may not be used to fulfil our Mitzvah of eating Matza.
The Gemara discusses the extent to which Matzah must be watched for this special purpose. One opinion suggests that it is sufficient if the dough is kneaded for this special purpose. And the Gemara concedes that there is no proof that Matzah must be watched from before kneading. Nevertheless, Rava taught that when cutting the grain in the fields and tying it into bundles, it should be kept in mind that this is for the sake of the Mitzvah of Matzah.
What is the Halachah?
The RIF rules that the Shemirah must be done from the time that the grain is cut.
The SHE'ILTOS (Parshas Bo) writes that from the moment the wheat comes in contact with water, it must be watched from becoming Chametz.
The ROSH (2:26) rules that it is adequate to have Shemirah from the time of kneading. He adds that where he lives the mills are water driven and water is known to splash into the area where the flour is ground. Therefore it is appropriate to watch the wheat from the time that it is ground into flour. Furthermore,even the Rif who suggest that the flour should be watched before kneading, means only that it is preferable but not that it is critical.
The RAN argues that one must watch from before kneading which can only mean from a time when there is a risk of it becoming Chamets, grinding.
This appears to be the opinion of RASHI in Chulin (7a) as well, who mentions that we watch the flour from the time of grinding.
All three opinions are cited by the SHULCHAN ARUCH (453:4). We therefore do our utmost to have the wheat watched from harvest. If such flour is not available, then flour that has been guarded from the time of grinding, or at least from whenever it comes near water. In extenuating circumstances, one may fulfill the Mitzvah with Matzah that was watched only from the time of kneading.