Guarding the matzot to be used for fulfilling the mitzva of eating matza on the Seder night has two implications: being extra vigilant to prevent it from becoming ĥametz and having the intention that the matza is to be used for the mitzva (“le-shem matzat mitzva”). It is therefore necessary that the matza be kneaded and baked by Jews who are halakhic adults, who can be relied upon both to guard against fermentation and to keep in mind that their actions are for the sake of matzat mitzva. One may not employ gentiles, minors, or mentally unfit workers to knead or bake matza, since they cannot be relied upon to have the proper intent (She’iltot, Rashba).
Some dispute the second implication above. In their opinion, the mitzva of guarding the matza requires extra vigilance to prevent the matzat mitzva from becoming ĥametz, but requires no special intent while doing it. Thus, gentiles and minors are fit to bake matzat mitzva, as long as an adult Jew supervises their work, ensuring that they work with alacrity and that the dough does not become ĥametz (Ra’ah).
In practice, at the time of kneading one should take care to fulfill both meanings of “guarding.” Thus, one must insist that Jews knead and bake the matzat mitzva, taking care that the dough does not become ĥametz and intending that the matza be for the sake of the mitzva. However, at the time of harvesting and grinding, the first meaning is sufficient. Thus, the wheat may be harvested and ground by gentiles as long as a Jew stands nearby and supervises their work, ensuring that it does not become ĥametz (SA 460:1; MB and SHT 4 ad loc.).
Le-khatĥila, one should say out loud, when starting to work on matzot, that all the work is being done for the sake of producing matzat mitzva; be-di’avad, it is sufficient that he thought it (BHL 460:1, based on Pri Megadim). He should also have in mind that it is for the matzat mitzva that one eats on Seder night, but if he had in mind that it is for matza for Pesaĥ, he has fulfilled the obligation (SAH 453:14).
According to Baĥ and Eliya Rabba, those who believe that the guarding must begin at the time of the harvest (see the previous section) also require a Jew to harvest and grind the wheat le-shem matzat mitzva. Some are meticulous about this in practice. (According to those who maintain that turning on a machine does not constitute proper intent – see the next section – the harvesting and grinding must be done by hand, not with agricultural machinery; very few people insist on this.)
Taz explains that the harvesting and grinding need not be done with the intention to use the wheat for a mitzva; therefore, one may have a gentile harvest and grind the wheat, provided that a Jew supervises and ensures that the wheat does not become ĥametz. Intentions to use the wheat for the mitzva are only necessary from the time of kneading; therefore, a Jew must be the one to knead and bake the matza (according to most poskim). Consequently, SA 460:1 rules that gentiles may not perform the kneading or baking of the matza. BHL 460:1 s.v. “ein” states that the universal custom is to be lenient in accordance with Taz to allow gentiles to harvest and grind the wheat under the supervision of a Jew. This is also the opinion of Ĥok Yaakov and SAH 453:16. Matza that is made this way is called “shmura mi-she’at ketzira” – supervised from the time of the harvest. See Hilkhot Ĥag Be-ĥag 16 n. 19, which beautifully explains Taz to mean that there are two types of guarding.