The process of purification for a Nida (menstrual woman) requires a period of "seven clean days" during which no blood is discovered, and this period must be preceded by a "Hefesek Tahara" examination to establish the cessation of blood flow. This examination entails the insertion of a "Moch Dachuk" ("inserted cloth") into the woman's body several minutes before sundown, which is removed some 35-40 minutes after sundown. The woman then inspects the cloth to ensure that her blood flow as indeed stopped.
The question arises as to whether a woman may walk in a public domain on Shabbat while the "Moch Dachuk" is inserted. Do we consider the woman to be "carrying" the cloth, which is forbidden in a public domain on Shabbat, or do we view the cloth as part of her attire?
The Gemara in Masechet Shabbat (65) writes that a woman may walk in a public domain on Shabbat with a "Moch." However, the Rishonim (Medieval Talmud scholars) disagree in identifying the kind of cloth to which the Gemara here refers. According to Rashi (classic Biblical and Talmudic commentator, France, 1040-1105), the Gemara speaks here of a cloth intended to protect the woman's garments from bloodstains. Tosefot (commentary to the Talmud by students of the Tosafist schools of Medieval Europe), based on an earlier passage in Masechet Shabbat, disagrees, contending that Halacha would forbid walking in a public domain on Shabbat with a cloth that serves merely to protect one's clothing from stains. According to Tosefot, the Gemara refers to a case of a woman with a steady flow of blood which would cause her discomfort if it touches her body. She therefore wears this cloth to protect her body, not her clothing, and the Sages allowed walking in a public domain on Shabbat with such a cloth to spare the woman the discomfort she might otherwise experience. Cloths intended to protect merely one's garments, however, may not, according to Tosefot, be worn in a public domain on Shabbat.
The Shulchan Aruch (303:15) follows Tosefot's view, permitting a woman to walk in a public domain only with a "Moch" that serves to protect her body from blood. Accordingly, the "Moch Dachuk," which is inserted once the woman's flow has stopped, when there is no longer any risk of blood flowing onto her body, may not be worn in a public domain on Shabbat. This is, indeed, the consensus of the major Halachic authorities, including Chacham Ovadia Yosef, Rabbi Moshe Halevi (in his work Menuchat Ahava), Rabbi Moshe Feinstein (Russia-New York, 1895-1986), the Be'er Moshe (Rabbi Moshe Stern of Debereczyn) and Minchat Yitzchak (Rabbi Yitzchak Weiss, Romania-England-Jerusalem, 1902-1989).
Chacham Ovadia Yosef does, however, permit walking in a public domain with a "Moch Dachuk" during "Bein Ha'shemashot" on Friday evening, the period of 13.5 minutes after sundown. The period of "Bein Ha'shemashot" on Shabbat is generally treated more leniently, and in this instance we may therefore take into account Rashi's position, which would permit walking in a public domain with a "Moch Dachuk" on Shabbat. Furthermore, some authorities were of the opinion that a "Moch Dachuk," which is inserted inside the woman's body, differs from the cloth described in the Gemara, which was worn outside the body and can thus be seen as being "carried" as the woman walks. Although we generally do not follow this opinion, during the period of "Bein Ha'shemashot" we may rely on this view and allow walking with a "Moch Dachuk" in a public domain.
Summary: A woman may not walk in a public domain on Shabbat while a "Moch Dachuk" is inserted, but she may walk in a public domain with a "Moch Dachuk" during the period of "Bein Ha'shemashot," meaning, for 13.5 minutes after sundown on Friday afternoon.
