We usually associate ancient wine presses with groups of people treading on grapes with their feet.
There were, of course, other types of wine presses, and it is important to understand how having a non-Jew operate them would affect the status of the wine that is produced.
The Gemara on today’s daf (=page) discusses a me’atzra zaira – a wine press where the grapes are squeezed by a board or by a beam that is pressed on them by a non-Jew. Rav Papi permits such wine, while Rav Ashi – some say Rav Shimi bar Ashi – forbids it. The Gemara explains that all agree that if the press was operated directly by the non-Jew, then the wine would be prohibited. The disagreement comes up only in a case of ko’ah koho – where the contact of the non-Jew comes from secondary or indirect action.
Rashi, who was, himself, a vintner, explains the case of me’atzra zaira to be a wine press where the juice was squeezed from the grapes by means of a beam pressed down on them by means of a screw, which was the method that was used where Rashi lived. Rabbenu Tam argues that based on the Gemara here, as well as in other places, it does not appear that a beam was used to squeeze the juice out of grapes, rather this method was used to squeeze juice out of the grape pits after the initial squeezing had already been done. The Ra’avad suggests that this case is where the grapes were not squeezed by means of stomping them, rather a board was placed on them in order to squeeze out the juice. The case of direct action (koho) would be when the non-Jew stood directly on the board; the case of indirect action (ko’ah koho) would be when the non-Jew rolled or pressed a beam on this board to squeeze the grapes.