Makhshirin, Chapter Five, Mishnah Six
1) One beats upon a hide:
a) If outside the water, it comes under the law of if water be put;
b) Inside the water, it does not come under the law of if water be put.
2) Rabbi Yose says: even inside the water it comes under the law of if water be put, because his intention was that the water should come off together with the filth.
Section one: The hide referred to in this mishnah was laundered, probably in a large body of water, and now the person is beating on it to get the dirty water out. If he does this outside of the body of water, then the water that sprays out does cause susceptibility, because he wants this water to come out.
But if he stands in the water and hits on the hide, then he doesn’t really care if the water comes out, because new water will come in. In other words, standing outside the water is a way to get it dry but standing inside the water will not make it dry.
Section two: Rabbi Yose says that even if he is standing inside the water, the water that splashes out causes susceptibility because he does want that water to come out of the hide in order to clean the hide. Thus even though the hide remains wet, the water that comes out is something he wants.