After a clean hefsek tahara, before sunset, the woman inserts a second bedikah cloth into the vaginal canal. This is cloth is called a moch dachuk. The cloth remains inside until after nightfall. The cloth is then removed and examined for any red stains. If the cloth is clean, she is assured that her period has completely ended and she can start shivah neki’im.
The moch dachuk does not need to be done with a bedikah cloth. One may use a tampon for the moch dachuk.
If a woman forgot to do a moch dachuk, she may still begin counting shivah neki’im. If she remembered to do the moch dachuk only after sunset, she does not need to insert one then.
One who’s fifth day after her period falls on a Friday, and accepts Shabbat upon herself early, should do the moch dachuk close to sunset. This is true even though the bedikah is done before accepting Shabbat.
If a woman’s vaginal lining is sensitive, or if she finds the moch dachuk particularly uncomfortable, she can consult with a rabbi, as there are some leniencies regarding the moch dachuk. A rabbi may permit her to skip this step of the tahara process.
A woman who sees a ketem in middle of shivah neki’im and had to start her shivah neki’im over does not need to do another moch dachuk.