Keilim, Chapter 25, Mishnah 8. The mishnayot here have been dealing with the outer and inner surfaces of a vessel and the fact that, for the purposes of tumah, they can be treated as two separate entities: the outside of a cup can be tamei while the inside remains tahor. Toward the end of the chapter, Rabbi Yose taught that there is a leniency when it comes to hands that are tahor. Our mishnah now asks "ketzad," how so, and spells out exactly what that leniency looks like in practice.
Holding the Cup by Its Grip
The first situation opens with "Hayu yadav tehorot," his hands are in a state of taharah, while "achorei hakos," the reverse side of the vessel, is "teme'im," tamei, since a tamei liquid had come upon it. The mishnah then describes what he does: "achazo bevet tzeviato," he lifts the cup by its tzevia, the handhold, the spot on the vessel fashioned to be gripped.
And the halachah is "eino choshesh," he need not entertain the possibility "shema nitme'u yadav," that his fingers picked up tumah "be'achorei hakos," from the vessel's outer side. Because the cup has a spot built for holding, we take it for granted that his fingers rested there and nowhere else. Having touched only the handhold, they never reached the impure exterior, and their taharah is undisturbed.
Drinking From the Cup
Now the second situation: "Haya shoteh bekos she'achorav teme'im." Someone is drinking from a vessel whose outer side carries tumah, once more because a tamei liquid had touched that surface. And the halachah is "eino choshesh," he need not entertain the possibility "shema nitma hamashkeh shebefiv," that the drink now in his mouth contracted tumah "be'achorei hakos," from the vessel's exterior, "vechazar vetimme et hakos," and then went back and conveyed tumah to the cup itself.
Consider the string of doubts one could raise here. While he drinks, perhaps the liquid on its way to his lips ran along the impure outer side of the vessel and took on tumah right there. That liquid is now sitting in his mouth. And perhaps a single drop of it slid back down, and that drop, itself tamei, would convey tumah to the vessel's inner surface and to whatever drink still remains within.
The mishnah releases him from that entire line of reasoning. He is permitted to assume he drank the way people ordinarily drink, with nothing running along the impure exterior, and so the interior of the vessel, together with all that it holds, retains its taharah.
The Point of the Mishnah
Both halves of the mishnah share a single theme, and together they illustrate Rabbi Yose's leniency. Where a person's hands, or the contents of the cup, are presently tahor, we do not construct speculative scenarios in which tumah might have been transferred. We assume the normal, expected course of events: a cup is held by its grip, and a cup is drunk from cleanly. That is the practical force of the leniency the mishnah set out to explain with the word "ketzad."