פירוש על עבודה זרה 110:3

According to R. Huna, as soon as the juice from the wine press begins to flow down into the vat, it is subject to the laws of yayin nesekh. What this would mean is that if a non-Jew touched the wine in the press as it started to flow down into the vat, all of the wine in both the press and vat would become prohibited. In the Middle Ages, non-Jews were almost always needed to press the grapes due to the large amount of wine they needed to produce and the small number of Jews available to do the work. How Jews worked around this prohibition was the subject of a fascinating study by Haym Soloveitchik.

According to the mishnah, the wine in the winepress does not become yayin nesekh, assumedly under any circumstance, even after it starts to flow. Therefore, one may purchase a winepress from a non-Jew even though he had put his hand into it.

The only way for the wine in the winepress to avoid becoming yayin nesekh (if touched by a non-Jew) is for the press to be stopped up such that the wine does not flow to the vat below. For once the wine flows, it can become yayin nesekh.