For the source text click/tap here: Chullin 12
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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.
We explore the notion of the presumption of expertize in schechita and other halachot.