Rav Naḥman quotes Rav as teaching that if someone watches a person perform sheḥita – ritual slaughter – from the beginning to the end, the animal can be eaten, but if he does not see the entire process, it cannot be eaten. The Gemara explains that if the slaughterer was a known professional there is no need to watch him throughout, and this statement refers to someone who was not knowledgeable in the laws of sheḥita. I might have thought that if we see him perform half of the slaughter properly we can assume that the rest was done properly, as well, so Rav Naḥman teaches us that we cannot make that assumption and must watch the entire process.
The clear implication from this ruling is that ritual slaughter performed even by someone who does not know the laws of sheḥita is acceptable if he is watched from beginning to end. Tosafot is troubled by this conclusion, given that an earlier Gemara (daf 3b) teaches that if we are unsure whether or not a person is expert in the laws of sheḥita, his slaughter cannot be relied upon, even if he is watched by another.
The Tosafot HaRosh distinguish between the two cases, arguing that the earlier Gemara was discussing a situation where someone happened to see the sheḥita, while the case on today’s daf was a situation where the person was specifically supervising the inexperienced slaughterer. Rabbi Akiva Eiger suggests that in the case on today’s daf we know for certain that the slaughterer was not expert in the laws, which is why he was being carefully watched, while in the earlier case we were simply unsure whether or not he was an expert, so people may have relied on him, and it was not enough that others were watching.