Siman 196 is the chapter in which halachah spells out exactly how a woman moves from bleeding to taharah: hefsek taharah, moch dachuk, the seven clean days, and immersion.

Today every woman who experiences uterine bleeding must count seven clean days, as the Torah says of the zavah:

"And when she is cleansed of her discharge, she shall count for herself seven days, and after that she shall be pure."

This siman sets out exactly how a woman does that.

The start of the process — the hefsek taharah

Before a woman begins counting seven clean days, she must perform a hefsek taharah, as the Shulchan Aruch states.

The reason a woman must perform a hefsek taharah is to confirm that the bleeding has stopped entirely. A woman becomes temei'ah the moment blood leaves the uterus, before she sees it externally. So even if she thinks the bleeding has stopped because she no longer sees blood externally, if blood is still leaving the uterus she cannot begin counting seven clean days. The hefsek taharah lets her confirm that blood is no longer leaving the uterus.

Biblical or rabbinic — a dispute

There is a machloket whether the obligation to perform a hefsek taharah is biblical or rabbinic. The dispute may exist because, unlike the obligation to count seven clean days, the obligation to perform a hefsek taharah does not appear explicitly in the verses.

In practice, halachah treats the hefsek taharah as a biblical obligation. If a woman did not perform one, her seven clean days and her immersion are invalid, and she must go through the entire process of taharah from the beginning.

So why does the question matter?

It has two main practical applications.

First — the Chatam Sofer's alternative

In an extreme case a woman may be permitted to use the Chatam Sofer's alternative to a hefsek taharah. I will not explain its details, because only some poskim accept it, and those who do accept it only in very extreme circumstances. It cannot be relied on without an explicit ruling from a rav.

Worth knowing: if a woman has a wound and cannot achieve a hefsek free of blood, or if performing a hefsek is impossible for some other reason, she should not assume all hope is lost. She should turn to a rav or a yoetzet for guidance.

This is in fact a key principle across the whole body of halachah, and especially in taharat hamishpachah: if you are trying to keep the halachah and something is not working, ask. As Masechet Avot puts it:

"The bashful one does not learn, and the short-tempered one does not teach."

Meaning: there is no need to be embarrassed to ask questions. That is how a person learns. (And if the one you ask is annoyed at being troubled with questions, go find another teacher — "the short-tempered one does not teach." A teacher who is too severe is one whose instruction does not land.)

Second — ben niddah

When a child is born to a woman who conceived while she was a niddah, although this has no halachic consequence for the child's status, the Gemara tells us the child is affected spiritually and holds the status of a ben niddah, which is associated with negative character traits.

In certain circles this means baalei teshuvah are not considered for shidduchim, out of concern that this spiritual status affects their character adversely.

Rav Moshe Feinstein zt"l wrote a remarkable responsum on this. He said that if we see that these baalei teshuvah in fact have fine character traits, we may assume they are not in the category of ben niddah at all.

How can that be? Their mother did not keep taharat hamishpachah. She was certainly a niddah!

He explains that the mother certainly immersed in an ocean or another kosher body of water without intent before conceiving. But what about a hefsek taharah? Here, says Rav Moshe, we see the force of the poskim who hold that a hefsek taharah is rabbinic.

Of course: Rav Moshe was speaking in the framework of finding merit, and did not say that in practice we may treat the hefsek taharah as rabbinic.

Practical halachah

Before beginning to count seven clean days, a woman must perform a hefsek taharah.