פירוש על נדה 3:2

"Not like the words of this (Hillel)" - that is overly stringent, for the walls of the uterus would not hold [the blood] in for so many days.

"Not like the words of this (Shammai)" - that is overly lenient.

"Rather, a twenty four hour period can reduce (a longer than twenty four hour period) between two checks (i.e. bedikot)" - For if there are many days in between two bedikot, we do not declare all the taharot [handled] in the days between the two bedikot ritually impure, but just those [taharot] that she dealt with during the previous day at this hour (i.e. twenty four hours prior to her seeing blood). And the reasoning of the Rabbis is explained later on in the Gemarah. It appears that a twenty four hour period can reduce [a longer amount of] time in between two bedikot.

"...and a period in between two checks (i.e. bedikot) (that is less than twenty four hours) can reduce a twenty four hour period." - For if the time in between two bedikot is less than a twenty four hour period, we go after it (the smaller amount of time in between the two bedikot) in order to be lenient. For example, if she performed a bedikah in the morning and found herself to be ritually pure, and she again performed and bedikah in the evening and found herself to be ritually impure - [as is mentioned] later on in the Mishnah (Niddah 11a) "Sometimes she is required to perform a bedikah in the morning as well as at twilight..." - we do not declare the taharot of the previous night ritually impure, since she performed a bedikah in the morning and found herself to be ritually pure. And we cannot say that there was blood present at that time (i.e. the previous night). Rather, those taharot from time time [after] the morning bedikah and onward we make ritually impure - which is the time between the two bedikot.