Machshirin, Chapter 1, Mishnah 5. This tractate deals with the liquids that make food susceptible to receiving tumah. Water alone does not accomplish this; the water has to come onto the food in a way that the owner is pleased with, and such water is called "b'chi yutan." Water that arrives against the owner's wishes leaves the food exactly as it was, unable to become tamei. This mishnah looks at a person who is deliberately getting water out of something, and asks whether that water counts.
The first case: "hamemachek es hakrisha," one who presses down on a leek. Water or dew has collected inside the leek, and he squeezes it in order to force that moisture out. The second case: "v'hasocheis se'aro u'chsuso," one who wrings out his hair or his garment. He was walking outdoors in the rain, his hair and his cloak became soaked, and now he is squeezing them so that the water should come out.
"Rav Yosi omer: hayotz'in b'chi yutan." Rav Yosi rules that the water which actually emerges is b'chi yutan. The reason is straightforward: getting that water out is precisely what the person set out to do. It is leaving with his full approval, so if it lands on food, it renders that food machshir l'kabel tumah.
"V'she'bo einan b'chi yutan." But whatever water stays behind, still sitting inside the leek, or still held in his hair or in the fabric of his garment, is not b'chi yutan. "Mipnei shemiskavein sheyeitzei mikulo": his whole intention was that all of the water should come out. The moisture that failed to emerge is not something he has any use for or interest in. It is not water he wants, so it carries no power of hechsher.
The Rav points out that this follows the same logic that appeared earlier in the chapter in the position of Beis Hillel regarding shaking a tree. There too, the water that a person shakes off the tree is water he wanted to come down, and it is therefore b'chi yutan: should it fall onto food, it makes that food machshir l'kabel tumah. The water that remains clinging to the branches after the shaking, however, was never wanted and never counts. Here in our mishnah the same distinction is drawn on the person himself and on his produce: what he succeeded in expressing is b'chi yutan, and what stubbornly stayed in place is not b'chi yutan and does not make anything machshir l'kabel tumah.