Machshirin, Chapter 5, Mishnah 8. Throughout this chapter the Mishnah keeps returning to one question: when water comes onto food, was the owner pleased with that water or not? Only water that a person wants can be a hechsher, a preparation that makes food able to receive tumah. Our mishnah applies that principle to two everyday coverings.
Two items are named here. The first is "kisui shel shulchanos", the cloth draped over tables, and the second is "shifah shel leveinim", the sheet stretched across bricks. Concerning both of them the mishnah rules "einam b'chi yuttan": whatever water reaches them did not arrive with the owner's approval. But the mishnah continues, "ni'aru, b'chi yuttan": should he shake those coverings out, the water is now treated as water that came with his approval.
The Rav explains the first case. People are eating at a table, and food is lying on it. A cloth is placed over the food to keep it from getting wet or soiled. Now the rain falls, or water splashes, and it lands on that cloth. The second case is similar in spirit: a person is in the middle of a building project, his bricks are laid out and not yet finished, and he covers them with a sheet so that they should not get soaked.
In both situations the water that lands on the covering is water the owner never wanted. He did not put the cloth there in order to catch water; he put it there precisely so that the water should not reach what is underneath. This follows the same logic we met in earlier mishnayos of the chapter, where something happens to be collecting rainwater although the owner has no interest in that water at all. He is not gathering it; he is guarding his food or his bricks from ruin. Water like this carries no ratzon, and therefore it is not b'chi yuttan and gives no hechsher.
The mishnah then qualifies its ruling with the words "ni'aru, b'chi yuttan." Should the owner shake out these coverings, the water on them turns into b'chi yuttan. According to the Rav, the shaking here is done for the sake of washing the covering itself: the owner makes the puddled rainwater do the job of cleaning his cloth. By drafting that water into his service he has demonstrated satisfaction with it, and from then on it ranks as water that arrived with his approval, capable of functioning as a hechsher l'kabel tumah.
The lesson of the mishnah is how little it takes to turn unwanted water into wanted water. The very same drops that had no halachic standing at all while they sat on a protective cover acquire full status the instant the owner decides to make use of them.