Parah, Chapter 6, Mishnah 1. Wherever our tractate speaks of kiddush, it means one particular act: dropping the ashes of the Parah Adumah into water that has already been drawn, so that the water becomes the mei chatas fit for sprinkling. Our mishnah begins a run of halachos governing exactly how that act is carried out, and the principle it establishes is that a person, and not gravity or chance, must be the one who brings the ashes into the water.
The Mishnah's terms follow from the discussion of vessels: shoket, trough, is the vessel in which the kiddush is performed and which holds the water, and shefoferes, spout or tube, is the small container in which the ashes themselves are kept.
A person, not an accident, must bring the ashes in
"Hamekadesh venafal hakiddush al yado o al hatzad, ve'achar kach nafal al hashoket, pasul": a man was performing the kiddush, and some of the ashes fell onto his hand or onto the side of the vessel, and only from there dropped down into the trough. The kiddush is invalid. "Nafal min hashefoferes lashoket, pasul": and so too if the ashes tumbled on their own from the spout straight into the trough, it is pasul.
The reason is that the Torah speaks of giving the ashes into the water, which means that they must enter the water through human power. If they slipped in from the vessel by themselves, or if he was holding them and a friend jostled him or the wind pushed his hand so that they fell in, the water is not sanctified.
Melachah invalidates the water, not the ashes
"Natal mishefoferes vechisah": he lifted the ashes out of the spout and then put a cover on the vessel; "o shehegif es hadeles": or he shoved a door closed. Only afterwards did he deposit the ashes in the water. The Mishnah now draws a fine line between two separate questions, ruling "hakiddush kasher vehamayim pesulin": the act of kiddush stands, but the water may not be used.
Melachah, engaging in unrelated labor, disqualifies the water, but it does not disqualify the ashes. And melachah has that power only up until the ashes have been placed into the water; from the moment the ashes are in, work no longer renders the mei chatas unfit. So in our case the labor he performed before putting in the ashes ruined the water, while the act of kiddush he then performed with the ashes was in itself a proper one.
Standing the spout upright
"Zekafah ba'aretz, pasul": if he stood the spout upright on the ground, the water is pasul, since that counts as a melachah. "Letoch yado, kasher": but if he stood it upright into his own hand, the water remains fit. "Mipnei she'efshar": because this is something he can do.
Others understand the final phrase in exactly the reverse sense, reading i efshar, meaning that no alternative is open to him: he has no other way of handling the ashes, for if he does not rest the tube upright against his palm the entire contents will pour out and be lost. An act he cannot avoid, and which is part and parcel of the kiddush itself, does not carry the status of melachah at all, and the water stays fit for use.