Makshirin, Chapter Two, Mishnah Two
1) If one shook a tree in order to cause food or something unclean to fall down from it, [the rain water dropping down from it] does not come under the law of if water be put.’
2) If [he shook the tree] in order to cause liquids to fall down from it:
a) Bet Shammai says: both [the liquids] that fall down and [the liquids] that remain [on the tree] come under the law of if water be put.’
b) But Bet Hillel say: [the liquids] that fall down come under the law of if water be put, but [the liquids] that remain [on the tree] do not come under the law of if water be put,’ because his intention was that [the liquids] should drop down from all the tree.
Section one: Since his intention was not to bring the rain down from the tree, but rather food or something else, such as an impure thing, the water that falls down does not cause produce to be susceptible to impurity. He doesn’t want this water to fall down.
Section two: In this case, he shook the tree in order to cause the rain water to fall down. Bet Shammai rules that even the water that remains in the tree now causes produce to be susceptible because he wanted that water to fall down. In other words, even if his intention is not fulfilled, the water still causes impurity because he wanted it.
Bet Hillel says that only the water that falls causes susceptibility because in the end, this is the water that he wanted.