Niddah, Chapter 1, Mishnah 2. The opening of our masechta established that a woman becomes temeiah as a niddah from the moment blood leaves her uterus, and it dealt with the question of retroactive tumah: since the blood may have been on its way for some time before she noticed it, the Chachamim ruled that she is considered temeiah retroactively for a mei'es l'es, a twenty four hour period, and that this period is shortened if she performed a clean bedikah, an internal examination, at some point within it. Our mishnah now adds two important qualifications to that rule.
A Woman Who Has a Veses
The first ruling addresses a woman of whom it can be said "yeish lah veses," that she possesses a veses: a settled schedule, so that her period arrives at the same point each month and her body behaves in a way that can be anticipated. About her the mishnah rules "dayah sha'atah": the hour in which she sees is enough for her. As far as questions of tumah and taharah go, nothing is pushed backward in her case at all. Her tumah begins at the actual sighting and not one moment sooner, for a woman whose cycle runs on a known schedule is not suspected of having started to bleed earlier without realizing it.
A Bedikah Performed at Tashmish
The mishnah's second ruling speaks of "hameshamashes b'eidim": a woman who, in connection with tashmish, inserts cloths for examination both beforehand and afterward. One might have thought that such a check, done for the needs of the moment rather than as a formal inspection, would carry less halachic weight. The mishnah teaches otherwise: "harei zo kipkida." Her examination has the standing of the pkida spoken of in the laws of retroactive tumah, no less than a bedikah performed deliberately for that purpose.
The practical consequence follows on both sides. It limits the mei'es l'es: if this examination was clean and took place within the twenty four hours preceding her sighting, the retroactive tumah reaches back only to that examination and no further. And it limits the reckoning of mipkida lifkida as well, since it now serves as the last clean examination from which we count.
Our mishnah thus softens the burden of retroactive tumah in two ways: for the woman whose cycle is fixed, there is no reckoning backward at all, and for any woman, an examination taken at the time of tashmish counts as a genuine bedikah that shortens whatever reckoning there is.