Although the hefsek taharah examination confirms that all uterine bleeding has ceased, there is a possibility that bleedng may recur between the examination and nightfall, when the shivah neki'im (the seven blood-free days) begin. The practice of inserting a moch dachuk (tightly packed soft cloth, commonly called a "moch") into the vaginal canal evolved in order to detect any traces of renewed uterine bleeding as a woman enters the shivah neki'im.
The moch dachuk is usually a bedikah cloth, though many authorities permit the use of a tampon instead. Some women insert two bedikah cloths (one directly atop the other), for added volume.
When there is any question about the validity of the hefsek taharah, a woman should ideally insert the subsequent moch dachuk with a little twist, so that it touches the circumference of the vaginal wall, and can then count as a backup hefsek taharah if necessary.
The moch dachuk should be inserted into the vaginal canal shortly before sunset, and then left inside, to be removed after nightfall (tzet hakochavim). This interval may be as short as 18 minutes, or may last for over an hour, depending on custom and geographical location. In order to avoid irritation, the moch dachuk should not be left in any longer than is customary. It is best to check with a local halachic authority to determine the appropriate amount of time that the moch dachuk should be left in.
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 dachuk, to prevent such irritation. Slightly dampening the moch dachuk with a drop of water to increase comfort may also be permitted.
The moch dachuk is evaluated after removal much like a bedikah, and any questions concerning colors on it should be brought to a halachic authority.
Forgetting or Omitting the Moch Dachuk
Ordinarily, if a woman performed the hefsek taharah examination but forgot the moch dachuk, her hefsek taharah remains valid, and she can still count the next day as the first of the shivah neki'im.
If a woman's vaginal lining is sensitive (e.g., due to perimenopause, hormonal contraceptives, or recent infections), or if she finds the moch dachuk uncomfortable or consistently finds stains on the moch dachuk, she should consult a halachic authority to determine whether the moch dachuk may be omitted.
If a woman will not have access to a washroom for an extended period around sunset (e.g., she will be traveling on a bus), she may omit the moch dachuk rather than leaving it in for significantly longer than halachically necessary.
During the process of first becoming tehorah following childbirth or a late miscarriage, a woman may omit the moch dachuk.
A bride need not insert a moch dachuk when becoming tehorah prior to her wedding.
If a woman performs a hefsek taharah on the same day she began bleeding (e.g., a new blood flow began during her shivah neki'im and ended that same day), the moch dachuk becomes more important. A halachic authority should be consulted if it was forgotten, or if she wishes to omit it. Some halachic authorities are stringent regarding the moch dachuk whenever the hefsek taharah is performed on a day on which she earlier experienced bleeding.