When slaughtering an animal, it is essential that the knife being used is perfectly smooth and has been checked to ensure that it is without nicks or notches. According to the Gemara on today’s daf ideally this examination should be performed by the resident Sage of the city – either, as the Ra’avad explains because it is considered a matter of halakhic judgment, given the possibility of an error, or because killing a living creature is a weighty issue, it should not be done without receiving permission from the city elder (Rabbi Zundel Kroiser in his Ohr haḤamah).
What if the knife is checked after the sheḥita – the ritual slaughter – has taken place, and is found to have a nick that would render it unkosher for use? Rav Huna says that even if after it was used for sheḥita it was used for breaking bones, nevertheless we must assume that the nick developed at the very beginning of the sheḥita, when the knife came into contact with the animal’s skin, and the animal is rendered unkosher. Rav Ḥisda argues that the animal is kosher, since it is likely that the nick in the knife was the consequence of breaking the bones that took place after the slaughter was complete.
The Gemara explains that Rav Huna’s argument is one that he has made before. Since the animal is not considered kosher for eating while it is alive, it retains its status until such time as we are certain that it was slaughtered properly. This statement appeared above (daf 9a ) in the context of his requirement to check the simanim – the windpipe and esophagus – immediately after slaughter, for without the verification we cannot assume that the sheḥita was done properly. The Ramban concludes from this that according to Rav Huna there is also a requirement to check the knife after each sheḥita, and if the knife is lost the animal cannot be considered kosher, even if we do not know that the knife had a problem. Rabbeinu Yona and the Rashba disagree, arguing that since the knife was checked prior to sheḥita we must assume that it was good at the time it was used to slaughter the animal. Thus it is only if the knife is found to be nicked that Rav Huna would forbid the animal.