Mikvaos, Chapter 6, Mishnah 2. This chapter is occupied with a single central question: when does water that is not itself valid mikvah water join together with a kosher mikvah and become part of it, and when does it remain a separate body of water standing on its own? A large part of the answer depends on the size of the passage between the two waters. Sometimes we require an opening the width of a shefoferes hanod, the spout of a wineskin, a measure of roughly three inches, and sometimes no such measure is needed at all. Our mishnah presents a case in which that requirement can be bypassed entirely.
Picture a situation that arises easily enough. Someone owns a dli, a pail, which has become tomei, and resting within it are additional keilim that have contracted tumah as well. Rather than immerse each item separately, he would prefer to finish the job in one motion: he lowers the pail into the mikvah while everything remains inside, expecting that the container and everything it holds will come up tohor together.
The mishnah rules: "Dli shehu malei keilim vehitbilan, harei eilu tehorim" - a pail that is filled with keilim, and he immersed them, they are all tohor. Notice that the mishnah says nothing about the pail's opening being of any particular width. The immersion works even though the mouth of the dli is smaller than a shefoferes hanod.
Why should that be? Because of a principle called migo. Since the immersion is genuinely accomplishing something for the outer kli, the pail itself, which is tomei and is now being purified by the mikvah water, the same act of tevilah is carried along to the keilim that are sitting inside the pail. Once the tevilah is effective for the container, it is effective for the contents as well, and we do not insist on an opening of a wineskin's spout.
Now the mishnah turns to the opposite case: "Ve'im lo toval" - if there was no immersion being performed on the pail itself. That is, the dli was tohor all along, so immersing it served no purpose whatsoever, and its owner is merely employing it as a handy container in which to lower the other keilim into the water. In such a case the argument of migo has nothing to stand on, since the tevilah accomplishes nothing for the pail. Consequently, "ein hamayim me'uravin ad sheyihyu me'uravin kishfoferes hanod" - we do not treat the water held in the pail as united with the mikvah's water unless the passage joining them measures a shefoferes hanod or more. Given an opening of that size, the water inside the pail takes on the status of mikvah water through its connection to the mikvah, and the keilim lying in that water have then undergone a valid tevilah.
The lesson of the mishnah is thus a neat one. When the tevilah has real work to do on the pail, the pail's contents ride along with it and no measured opening is required. When the pail is nothing more than a carrying case, the water inside it must earn its status as mikvah water in its own right, and that happens only through a connection the width of a wineskin's spout.