Taharos, Chapter 9, Mishnah 7. Our chapter deals with olives on their way from the tree to the press, and with the question of when the liquid oozing from them renders them susceptible to tumah. The governing principle throughout is that liquid creates hechsher only when the owner is content to have it; as long as a man is still gathering and adding to his heap, the sap that collects there is of no interest to him, his work is unfinished, and the olives are not muchsharin. This mishnah asks what happens when he changes his mind in the middle.
The mishnah now presents a case that arises in the middle of the process: "Ratzah litol mehen", the owner wishes to take some of the fruit off the pile, "bad echad", an amount equal to a single load for the press, "o shenei badim", or double that amount, and to press it immediately, even though the rest of the crop is still being brought in. The question is whether a decision of this kind announces that the collecting phase has closed, so that the liquid pooling in the heap has now become desirable to him and the olives become muchsharin.
"Beis Shammai omerim: kotzeis betumah umchapeh betaharah." Beis Shammai permit him to cut away the pressing's worth of olives with a tamei implement, which is to say that the act of removing them does not affect the standing of the fruit; while he is taking off his bad's worth, the olives remain insusceptible. It is the covering of what remains that changes matters. Covering the heap is the signal that he has moved from collecting to pressing, and once that signal has been given, the olives themselves shift in halachic reality from insusceptible to susceptible. Therefore the covering must be done in taharah.
"Beis Hillel omerim: af mechapeh betumah." Beis Hillel are a shade more lenient. In their view even the covering may be done with tamei implements, because taking off a pressing's worth and beginning to process it has not in fact altered the status of the pile. So long as he still intends to add more olives, the entire operation is regarded as incomplete, and incomplete work means that the seeping liquid is still unwanted.
"Rabbi Yosi omer: chofer bekardumos shel mateches umolich leveis habad betumah." Rabbi Yosi speaks in deliberately extreme terms. Not only may the implement be tamei, he may dig into the heap with metal axes. A metal ax suggests a battle ax, an object carrying the severest grade of tumah imaginable, avi avos hatumah, and Rabbi Yosi says that even such a thing may be used. And he may carry the olives to the beis habad, the oil press, betumah. In Rabbi Yosi's opinion the olives do not move from insusceptible to susceptible until the work is genuinely finished, which makes him a more far reaching version of Beis Hillel.
The halachah follows Beis Hillel.