פירוש על עבודה זרה 143:12

In the first section a Jew pours his wine into the container of a non-Jew using a funnel. The liquid that remains in the funnel when the Jew pours the wine into the non-Jew’s flask has the same status as the wine in the non-Jew’s flask itself. The fact that the wine in the flask is yen nesekh, means that the wine in the funnel is as well. This is because the funnel goes into the non-Jew’s container and may “take back” some of the yen nesekh. If even a drop of wine should remain in the funnel and the Jew should then pour more wine into the funnel and give it to a Jew, all of the wine has become contaminated as yen nesekh and it is all forbidden.
In contrast, the second section teaches a different principle from the previous section. When one pours from one vessel into another, and the poured liquid never touches both vessels at the same time, there is no contamination from the lower vessel into the higher vessel. While the bottom liquid might become itself contaminated (if it is poured into a non-Jew’s flask), the top liquid remains permitted.