Machshirin, Chapter 4, Mishnah 1. Masechta Machshirin turns on a single question: when does water that comes onto something count as having arrived biratzon, with a person's approval, so that if it touches produce it renders that produce hechsher lekabel tumah, susceptible to receiving tumah? Water a person wanted is called bechi yutan; water he did not want leaves the produce as it was. Our Mishnah applies that test to a man bending down for a drink and to a man drawing water in a barrel.
Bending down to drink
"Hashocheh lishtos, hamayim ha'olim befiv uvisfamo bechi yutan, bechotmo uveroshov uvizkano einan bechi yutan." A person bends down over a water source to drink. The water that comes up on his mouth and on his mustache is bechi yutan, but the water on his nose, on his head or on his beard is not.
The Ra"v explains the distinction. When you drink, water on the mustache is a certainty; it rings the mouth, it is part of the drinking itself, and in that sense it is water you want, part of the satisfaction of the drink. The nose, the head and the beard are a different matter. That water is not the water you are drinking, and it is not water you have any interest in having there. Since it is unwanted, it is not bechi yutan, and produce it touches is not made susceptible.
Filling a barrel from a well
"Hamemaleh bechavis, hamayim ha'olim achareha uvachevel shehu mechunach al tzavarah uvachevel shehu letzorchah, harei zeh bechi yutan." Picture the setup: a wooden barrel, a length of rope wound around its neck to secure it, and a further rope tied to that one by which the barrel is lowered down into the well.
When the barrel is lowered in and drawn up, the water that comes up on the outside of the barrel, and on the rope wound about its neck, and on the lowering rope, is all bechi yutan. True enough, nobody actually desires a wet barrel or a wet rope. But once you lower a barrel into a well, that everything will come up dripping is simply understood, and water that is the inevitable companion of what you set out to do is water that came biratzon.
The Mishnah adds one qualification with the words "uvachevel shehu letzorchah": of the lowering rope, only the portion actually needed for the job is included. Water on the length of rope required to let the barrel down and draw it up is bechi yutan; water on the surplus rope beyond that is not, since there was never any need for that part to get wet.
"Kamah hu tzorchah? Rabbi Shimon ben Elazar omer: tefach." How much rope is counted as needed? A tefach, a handbreadth. So to lay out the whole picture: the barrel and the rope bound around its neck are entirely included, while of the rope used to lower it only a tefach is included, because that is all the rope the task calls for. That much wetting is expected and therefore biratzon; the rest, even if it happens to get soaked, is not.
Filling the barrel under a spout
The final clause changes the method: "Nesanah tachas hatzinor" - he placed the barrel under a spout and allowed the stream to fill it, rather than dropping it down a well. In that case the Mishnah rules "einan bechi yutan": whatever wets the barrel's outside surface and the rope is left out of the reckoning. The reason is that this way of filling demands no wetting on the outside whatsoever. To catch water from a pipe, the vessel's exterior need never be splashed and the rope need never be dampened at all. Any droplets that do land there arrive without serving any purpose of his, so they are not considered wanted, not biratzon, and produce they touch remains as it was.
The Mishnah thus draws a fine and very practical line. Water is wanted not only when a person deliberately applies it, but also when it is the unavoidable accompaniment of what he deliberately did, and no further: not on the parts of the face that drinking never required, not on rope beyond the handbreadth the drawing required, and not at all when the way he filled his barrel never required it to be wet in the first place.