Machshirin, Chapter 5, Mishnah 4. The whole masechta turns on a single word in the Torah's phrase "ki yutan mayim al zera", that water renders produce susceptible to tumah only when it has been placed there willingly. Water still joined to the ground cannot make anything a candidate for tumah; the question always begins the moment water is detached, and the question is always the same: did this water come off with the owner's consent, beratzon, so that it counts as bechi yutan? Our mishnah applies that test to a man measuring a cistern.
Let us take the wording piece by piece. The first opinion belongs to Rabi Tarfon: "Hamoded es habor", one who takes the measure of a cistern, "bein le'omko", whether he is measuring how far down it goes, "bein lerochbo", or how far across it stretches, in both situations "harei zeh bechi yutan", the water counts as having been put there willingly. Rabi Akiva then disagrees: "le'omko bechi yutan", for the depth it is indeed bechi yutan, "ulerochbo eino bechi yutan", but for the width it is not.
Picture how measuring was done. A person had no tape measure; he used his own hand or arm, or he lowered a reed into the pit. Either way, when he pulls the hand or the reed back out, water comes out clinging to it. That water is now detached from the cistern, and so it becomes a live question: is it considered water that came out willingly, making it fit to be machshir, to prepare food for tumah?
Rabi Tarfon rules that it makes no difference what he was measuring. Whether he was checking how deep the cistern is or how wide it is, the water that came up on the hand or the reed is bechi yutan, and it can therefore render food susceptible to tumah.
Rabi Akiva draws a line between the two measurements. Measuring depth is bechi yutan; measuring width is not. His reasoning follows the nature of the act itself. There is no way on earth to measure how deep the water is without lowering your hand or your reed into the water. Getting wet is not an accident of the process, it is the process. And since the wetting is built into what he set out to do, we regard it as done with his consent, even though he has no interest in the water on his arm as such. Measuring width is different. In principle a person can stretch his reed across the mouth of the pit, above the water line, and never touch the water at all.
Now, the case the two Tannaim are arguing about is precisely where that theoretical option is not available: a cistern so full that the only practical way to take its width was to reach into the water and come out wet. Rabi Tarfon says that here too the man knew perfectly well that his hand would come out wet, so the water is beratzon and it is bechi yutan. Rabi Akiva answers that we judge by the ordinary character of the act, not by this particular pit. Since measuring width does not normally require contact with the water, the wetting is not folded into the act, and even here the water is not bechi yutan.
The halacha follows Rabi Akiva: water that comes up while measuring the depth of a cistern is bechi yutan and is machshir; water that comes up while measuring its width is not.