Parah, Chapter 7, Mishnah 7. This chapter is occupied with a single principle: while a person is engaged in drawing the water for the chatas, in sanctifying it with the ashes, or in sprinkling it, he may not busy himself with any outside labor. Any act that does not serve the sacred work itself counts as a melachah, and a melachah performed in the middle of the process renders the water pasul. Our mishnah applies this to a very ordinary motion: what a person does with the rope in his hand as he pulls the bucket up out of the well.
The words of the mishnah are these. "Hamechanein es hachevel", one who gathers up the rope, "al yad al yad", hand over hand, taking in one stretch of it after another as he goes, "kasher": the water he draws remains fit. "V'im kineno ba'achronah, pasul": but should he leave the winding for the very end, doing it once the work is over, the water is disqualified.
Picture the scene. A person lowers his pitcher into the water, and as he hauls the full vessel upward, the rope gathers in his hands. He winds it around his hand as he goes, a length at a time, in rhythm with the drawing. That winding is not a separate act at all; it is part of how a person raises water from a well. Since the coiling was needed for the collecting itself, it does not qualify as a melachah, and the water he draws is fit.
But if he first completed the drawing, so that the water was already collected and standing there, and only then took the rope and wound it up, that coiling no longer serves the drawing of the water in any way. It has become an act of its own, an outside labor performed during the sacred process, and the water is unfit.
A second opinion follows. Rebbe Yosi states: "v'zeh hechsheru hora'as sha'ah", this ruling of fitness was only a hora'as sha'ah, a decision issued in Yavneh for the pressure of that moment, not a permanent halachah. And Rebbe Yosi specifies precisely which situation that emergency measure covered. It applied here, to gathering the rope while the water is being lifted, and not to the ruling stated immediately before, where a person moved the rope past the place where it was actually needed. He fixes the temporary allowance to its own case, so that no one should stretch it to a case for which it was never granted.
The lesson of this mishnah is the fineness of the line. The very same motion, winding a rope around one's hand, is either part of the mitzvah or a foreign labor that spoils the water, and the only difference is whether it was still serving the drawing when it was done.