The hefsek taharah is a thorough internal examination to establish that bleeding has ceased. It must be performed before sunset, preferably in the late afternoon shortly before sunset, and is generally removed immediately so that the woman can check it. In the event that it is left in until after sunset, it is still valid.
The moch dachuk (commonly called a “moch” literally, tightly packed wadding) is inserted into the vaginal canal to detect any traces of renewed uterine bleeding after the hefsek taharah. 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 20 minutes, or may last for over an hour, depending on custom and geographical location.
A hefsek taharah is essential while a moch is generally only an accepted custom.
For more information, please see our articles on hefsek taharah and moch dachuk. Please write back with any further questions.