Commentary for Avodah Zarah 4:8

גת בעוטה – that the idolater had trampled and stamped.

A winepress [containing] trodden [grapes] may be purchased from a non-Jew even though it was he that lifted [the trodden grapes] with his hand and put them among the heap. And [the juice] does not become yen nesek (wine assumed to have been used as a until it descends into the vat.
When it has descended into the vat, what is in the vat is prohibited; But the remainder is permitted.
In chapter two, mishnah three, we learned that Jews may not drink wine touched by non-Jews, lest they had used the wine as a libation. The remainder of tractate Avodah Zarah will deal with the prohibition of non-Jewish wine. Our mishnah defines at what point in the process of wine-pressing do the grapes and grape juice begin to be considered wine.
Section one: A Jew may buy a winepress and all of the grapes that are being trodden in it from a non-Jew, even though the non-Jew has lifted up the trodden grapes and moved them into a different heap. At this point in their processing the grapes are not yet considered wine, and therefore their being handled by a non-Jew does not make them “yen nesek”, which is forbidden to Jews. In other words, while we might suspect that non-Jews offer up libations with wine, they will not do so with grapes that are not yet fully pressed, nor will they do so with grape juice. Only when the juice has descended into the vat where it will ferment into wine is it considered yen nesek.
When the wine does descend from the winepress into the vat, the wine which is in the vat is prohibited. However, that which remains above in the winepress is still permitted, even though it has been handled by a non-Jew.

אע"פ שהוא נוטל בידו – grapes from the wine and place them on the pile (i.e., the place on the altar where the ashes are piled up), the place of the gathering of the grapes that is made like a heap that is called תפוח/grape-heap, and our Tanna/teacher holds that it does not become libation wine until it flows down to the cistern and according to the hour, he becomes excited and returns and worships it. But this is the earliest version of the Mishnah and it is not Halakha, but rather, since the wine began to be drawn, it became libation wine.

מה שבבור אסור – if an idolater would come in contact with it afterwards.