פירוש על מקואות 2:6

המסנק (if one presses/scrapes up [the mud] against the sides of the pool) – like removing [to the corners] (see Tractate Shabbat, Chapter 20, Mishnah 4) – that he removes the mud to one side. But not that he detached the mud from the water, but rather within the water, he removed it to one side. For if he had removed it (i.e., the mud) from the water, it would be considered like it is drawn [water], and if they pulled from it three LOG, it invalidates the Mikveh. But this is specifically when there aren’t forty Seah in the Mikveh, this is when the three LOG of drawn water invalidates the Mikveh, but if there were forty Seah in it (i.e., the Mikveh), even all the drawn water in the world would not invalidate it.

If one scraped mud to the sides and then three logs [of water] were drawn out from it [from the mud], [the mikveh is still] valid. If one scraped some mud out of the bottom of a mikveh that doesn't have forty seahs of water in it, and then three logs of water dripped back into the mikveh from the scraped mud, the mikveh is still valid. The water that comes out of the mud is not considered to be "drawn water" such that it invalidates the mikveh.

ור' שמעון מכשיר – because he does not intend for drawing [water] but rather to remove the mud to another place. But the Halakha is not according to Rabbi Shimon.

But if removed the mud away and three logs were drawn from it [into the mikveh] it becomes invalid. In this case he removed the mud from the mikveh and didn't just scrape it away to the sides. If the water again seeps out of the mud and goes back into the mikveh it does disqualify it. Such water is considered to be "drawn water."

Rabbi Shimon pronounces it valid, since there was no intention to draw [the water]. Water that one doesn't intend to draw out from somewhere doesn't count as drawn water in order to disqualify a mikveh. Rabbi Shimon considers the water that comes out of the mud to be unintended because his intention was to draw out the mud, not the water. However, the first opinion in the mishnah considers this water to be drawn because there was no way for him to draw out the mud without also drawing out the water.