Machshirin, Chapter 5, Mishnah 6. This chapter is occupied with the question of when water counts as having come onto food with the owner's approval. Water only makes food susceptible to tumah when it arrives "bechi yutan," that is, in a manner that the owner is pleased with, and much of the chapter works through everyday situations in which a person gets something wet while his real interest lies elsewhere. Our Mishnah takes up such a case: a worker beating out a soaking wet animal hide.
"Hachoveit al hashelach chutz lamayim, bechi yutan." A shelach is a hide that still has its hair on it. Picture the worker: he has been rinsing the skin in a stream, and now he lifts a stick and strikes it so that the moisture the leather soaked up will be forced out. When he does the beating away from the water, the spray that flies off is bechi yutan. The reason is straightforward: the whole purpose of his blows is to expel that moisture, so its departure suits him perfectly. This parallels the earlier ruling about someone who shakes a tree in order that the water resting on it should fall. Water whose exit a person desires is water capable of serving as hechsher lekabel tumah.
"Vesoch mayim, einan bechi yutan." Now change one detail: the hide is still lying in the water while he beats it. Here the Mishnah rules that the water is not bechi yutan. Since the hide remains submerged, whatever water he knocks out will immediately be replaced by the water surrounding it; the hide is not becoming any drier. Clearly, then, his interest here is not the water at all. What he wants is to knock the dirt loose. The water is beside the point, and water that comes out under such circumstances cannot make anything susceptible to tumah.
"Rabi Yose omer: af sochamayim bechi yutan, mipnei shemiskavein sheyeitzei imah tzoah." Rabi Yose disagrees. Even when the hide is being beaten inside the water, he holds that the water is bechi yutan, because the man does have an interest in the water leaving: he wants the filth to go out together with it. The water in and around the hide has become dirty, and his aim is to push that soiled water out so that fresh, clean water can take its place and the washing can continue. Because he wants that water displaced, it is fit to serve as hechsher.
It is worth clarifying what exactly the two opinions are arguing about. If we actually know that this worker's plan was to drive out the dirty water and replace it, presumably even the Tanna Kamma would concede that the water is bechi yutan and makes food susceptible to tumah. The dispute is really about the ordinary, unspecified case, where we have no information about what was on the worker's mind. Rabi Yose holds that we assume the standard intention is to move the water out, so it is bechi yutan; the Tanna Kamma assumes the opposite, that a man beating a hide in the water is thinking only about the dirt.
As to the halachah, the Rav writes that we do not rule in accordance with Rabi Yose. A hide beaten while still lying in the water leaves us with water that is not bechi yutan and cannot serve as hechsher lekabel tumah.