The Mishna on today’s daf presents a situation where a knife falls and slaughters an animal. According to the Mishna, such slaughter is not sufficient and the animal is not kosher. The source for this ruling is the passage in Sefer (27:7) that teaches that a person must slaughter and eat. Only after you slaughter the animal are you permitted to eat its meat.
The case in the Mishna must be understood where the animal is lying on its side and the knife falls in such a manner that it cuts through the trachea and esophagus perfectly, with none of the problems with sheḥita (see above, daf 4) occurring.
The Gemara points out that the law of the Mishna is only true in cases where the knife fell on its own. If, however, the knife was directed by the person, then it is acceptable as ritual slaughter, even without specific intent to perform sheḥita.
According to the Rashba, it appears that the case of the Gemara that is acceptable is when the knife fell out of his hand, since we view it as being the act of an intelligent person – even if it was unintentional. A parallel case to this would be a case of damages, where a person who is sitting with a stone in his lap – even if he is unaware of the stone – will be held liable for any damage that it causes if it fell from him (see Massekhet Bava Kamma daf 26b ). Thus, the case of the Mishna must be when the knife fell because it was blown by the wind. Others (Ra’ah, Meiri) argue that sheḥita must involve the direct force of the individual, and it will only be kosher slaughter if the person actually causes the knife to fall. According to this approach, sheḥita differs from the case of damages, since all that is needed for a person to be held responsible is that he was the cause of the damage. Rashi goes one step further and suggests that even if there is no requirement to have specific intent to perform sheḥita, nevertheless he must minimally intend to make the knife fall. Otherwise it is no better than being blown off the table by the wind.