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 (=page) 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 halakhicjudgment, 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 haHamah).
What if the knife is checked after the shehitah – 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 shehitah it was used for breaking bones, nevertheless we must assume that the nick developed at the very beginning of the shehitah, when the knife came into contact with the animal’s skin, and the animal is rendered unkosher. (For reference, see schematic drawing of theanimal’s neck, showing the esophagus and windpipe directly beneath the animal’s skin, with the spine in the middle.) Rav Hisda 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 shehitah was done properly. The Ramban concludes from this that according to Rav Huna there is also a requirement to check the knife after each shehitah, and if the knife is lost the animal cannot be considered kosher, even if we do not know that the knife had a problem. Rabbenu Yonah and the Rashba disagree, arguing that since the knife was checked prior to shehitah 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.