Machshirin 1:1

If a liquid was welcome at the beginning and unwelcome at the end, or welcome at the end and unwelcome at the beginning, ki yutan applies to it. (Ki yutan is the rule that foods are rendered susceptible to impurity by having liquids put on them.) Ritually unclean liquids render foods unclean regardless of whether they are welcome or unwelcome.

Machshirin 1:2

If one shakes a tree in order to dislodge some fruit or something ritually unclean, ki yutan doesn’t apply to any water he might shake off. If one shakes a tree in order to extricate water from it, Beis Shammai say that ki yutan applies to what comes off and to what remains on the tree. Beis Hillel say that ki yutan applies to what comes off but not to what remains on the tree because he decides that its unusable.