Machshirin, Chapter 4, Mishnah 4. This chapter continues the central theme of the entire tractate: liquid gives food the ability to become tamei (hechsher lekabel tumah) only when it came onto the food with the owner's consent, as the pasuk says, "vechi yutan mayim," if water be put upon the seed. Water that arrives on its own, against a person's wishes, is not "bekiyutan" and leaves the produce exactly as dry, so to speak, as it was before. Our Mishnah asks what happens when a person has to deal with unwanted water that has already landed where he did not want it, and how he can get rid of it without triggering hechsher along the way.
The case
"Chavis sheyarad hadelef letochah": a barrel that a leak has dripped into. Picture fruit stored in a barrel while water trickles down from a leak above and collects inside. None of this suits the owner in the slightest; damp produce gains him nothing at all. Because the liquid arrived shelo biratzon, no hechsher has taken place, and the fruit still cannot contract tumah. What he is left with is a purely practical difficulty: he needs to be rid of the accumulated liquid, and he would like to drain it off without bringing hechsher upon his produce in the process.
Beis Shammai: break the barrel
"Beis Shammai omrim yishaber." The vessel, in their view, has to be smashed. Tilting it and letting the liquid run off is not an option they can allow: the instant he tips it, the water slides from one part of the container to another, and that sliding is precisely what he intends, since draining it is his entire purpose. Liquid set in motion by a person's own design is liquid moving with his approval, and whatever it passes over along the way is treated as having been dampened willingly.
This is the principle behind Beis Shammai's ruling: consent to the flow is sufficient. He may want no contact whatsoever between the liquid and the fruit, yet since the motion itself is what he sought, the wetting that results from that motion qualifies as bekiyutan, and the produce becomes fit to receive tumah. With no other avenue open to him, his only safe course is to break the vessel and let the liquid escape on its own.
Beis Hillel: he may pour it out
"Beis Hillel omrim ye'areh." Tipping the barrel, they say, is entirely acceptable. To their mind, the liquid sliding about inside the vessel carries no consent with it. Granted, he is fully aware that tilting the barrel will send water washing over the fruit; but awareness is not the same thing as desire. The contact comes as a side effect he would gladly do without, and whatever a person has no wish for is not biratzon. Nothing here creates hechsher lekabel tumah, and there is no reason to destroy a perfectly good barrel.
Where both agree
"Umodim she'im moshit yado venotel peiros mitochah, veheim tehorim." Here even Beis Shammai concede to Beis Hillel: if he reaches his hand into the barrel and lifts the produce out, the produce remains tahor. When he removes the fruit by hand, any water that happens to touch it is plainly not part of what he is trying to accomplish, and it is not even a necessary consequence of the act; the produce may well come out without water running over it. Removing the produce by hand therefore involves no consent to the water at all, and no hechsher results.
The lesson of the Mishnah is how finely the Torah measures a person's will. Two men empty the same barrel of the same unwanted water; the whole question is only how directly that water's contact with the food flows from what they actually wanted. Beis Shammai see the deliberate pouring itself as consent enough; Beis Hillel see it as an accident a person tolerates rather than desires. And both agree that a hand reaching in to rescue the fruit expresses no desire for the water whatsoever.