After the hefsek taharah examination, a woman inserts a bedikah cloth and leaves it inside the vagina from before sunset until after nightfall. This is called the moch dachuk.
Although the hefsek taharah examination confirms that all uterine bleeding has ceased, the remote possibility exists that it may recur between the examination and nightfall. Therefore, the practice of inserting a moch dachuk (commonly called a "moch" – literally, tightly packed wadding) into the vaginal canal has evolved to detect any traces of renewed uterine bleeding.
The moch is usually a bedikah cloth, although some authorities permit the use of a tampon instead. It should be inserted into the vaginal canal shortly before sunset, and may be removed after it is completely dark (tzet hakochavim). This interval may be as short as 18 minutes, or may last for over an hour, depending on custom and geographical location.
It is important that the moch dachuk not irritate the vaginal wall, as this can lead to stains from vaginal rather than uterine blood. Some women find it helpful to sit or lie down while using the moch, to prevent such irritation. If a woman's vaginal lining is sensitive (e.g., due to childbirth or the use of hormonal contraceptives), or if she finds the moch uncomfortable, she should consult a rabbi to determine whether the moch is necessary.
Ordinarily, if a woman performed the hefsek taharah examination but forgot the moch, her hefsek taharah remains valid, and she can count the next day as the first of the seven blood-free days. But if she performed the hefsek taharah on the same day that she commenced bleeding (i.e. she bled for only part of one day), there is a halachic presumption that her bleeding may recur even up to the last moment of that day. In such a case, the moch dachuk becomes crucial and a rabbi should be consulted if she forgets it. Some authorities extend this rule to any day on which she earlier experienced bleeding.
Users of Internet filtering services: This site discusses sensitive subjects that some services filter without visual indication. A page that appears 100% complete might actually be missing critical Jewish-law or medical information. To ensure that you view the pages accurately, ask the filtering service to whitelist all pages under yoatzot.org.