Mikvaos, Chapter 2, Mishnah 6. Alongside the invalidation of mayim she'uvim, drawn water, there is a second disqualification that the Torah teaches us from the pesukim: water that came into the mikveh through tefisa al yedei yad adam, water gathered by the intervention of human hands. Water that human hands collected and placed into the mikveh is likewise pasul. Whether this pesul is identical to that of drawn water or a category of its own is discussed among the poskim; for the purposes of this mishnah we will treat it as carrying the same problem as she'uvim. This mishnah presents a case in which a person handles the water of his own mikveh, and asks whether his handling turns it into water taken up by human hands.
Pushing the Mud Aside
A man has a mikveh whose water is thick with mud and dirt. He wants clear water in which to immerse, so he sets about separating the mud from the water. In the first case he does this entirely within the mikveh: he pushes the mud off to the sides, never lifting anything out of the basin, never uprooting the mud from the mikveh at all.
"Hamesaneik es hatit letzedadin, u'moshcho mimennu shelosha lugin, kasher." One who pushes the mud to the sides, and in doing so draws off three lugin of water from it, the mikveh remains kosher. The reasoning: this water was originally mei geshamim, rainwater that arrived in the mikveh without the intervention of a vessel or of human hands, and it is now sitting in the mikveh proper. When he shifts it about inside the mikveh itself, it never departs from the mikveh, and so it retains its status as valid water. Such manipulation is not considered tefisas yad adam.
Lifting the Mud Out
"Hayah tolesh u'moshcho mimennu shelosha lugin, pasul." If, however, he lifted the mud up out of the mikveh, and in the course of straining and draining it three lugin of water came off it, those three lugin are pasul. They had been separated from the body of the mikveh, and once they were held in his hands they acquired the status of water taken up by human hands, which is the equivalent of drawn water. When they fall back down into the mikveh, they invalidate it. Here too we are speaking of a mikveh that has not yet reached the threshold of forty so'im of rainwater, which is why three lugin have the power to spoil it.
Rebbi Shimon's Dispute
"Rebbi Shimon machshir, mipnei shelo niskaven lish'ov." Rebbi Shimon rules that even here the mikveh stays fit, since the man never meant to draw water at all, and so never meant to create tefisas yad adam. All he wanted was to clear the mud away from his water. If some water was caught by his palms only as a byproduct of that effort and then trickled back down into the mikveh, Rebbi Shimon does not classify it as water collected by a human being, and the mikveh is therefore valid.
The Mishnah thus draws a clean line: what stays inside the mikveh stays valid, and what is lifted out and returns is treated as though it had been drawn, with Rebbi Shimon adding that intent, and not only the physical act, determines whether human handling disqualifies.