• Relying on a כותי’s shechitah with יוצא ונכנס

Abaye says the Mishnah’s first phrase means that all may shecht (לכתחלה), including a כותי, but only כשישראל עומד על גביו – when a Yisroel stands over him to supervise, but יוצא ונכנס – if he goes out and comes in intermittently and does not see the entire shechitah, we do not allow him to shecht (because כותים do not hesitate to feed נבילה to Jews, since they do not accept that לפני עור לא תתן מכשול prohibits doing so). If he did shecht without constant supervision, we give him a כזית of the meat to eat; if he eats it, we are confident he shechted properly (for he would not eat it otherwise) and the meat is permitted (this is the intent of the Mishnah’s second phrase, that their shechitah is valid [בדיעבד]). Rava proves that "יוצא ונכנס" is sufficient supervision: a Mishnah permits leaving wine with an idolator, without constant supervision, provided he is יוצא ונכנס!? Rava therefore modifies Abaye’s interpretation of the Mishnah to fit with his opinion. Later, the Gemara defends Abaye’s position, differentiating between wine, which the idolator did not touch, and shechitah, which the כותי touched, and needs only a moment to invalidate the shechitah.

• A מומר לתיאבון may shecht לכתחלה if the knife is examined

Rav Ashi explains that the Mishnah means that all may shecht לכתחלה, including a ישראל מומר – Jewish renegade (one who regularly transgresses a prohibition). The Gemara clarifies that he refers to a מומר לאכול נבילות לתיאבון – to eat neveilah for enjoyment (i.e., when he cannot obtain kosher meat) about whom Rava ruled: בודק סכין ונותן לו – one examines a knife and gives it to him to shecht with, ומותר לאכול משחיטתו – and it is permitted to eat from his shechitah, since he would only eat kosher meat when it is available (however, we cannot rely on him to bother fixing or replacing a nicked knife). If the knife was not checked, he may not shecht, but if he did, we check the knife afterwards. נמצאת סכינו יפה – If his knife was found fine (without nicks), it is permitted to eat the meat (this is meaning of the Mishnah’s second phrase).

• Shechitah of "מומחין" and "מוחזקין"

Ravina first explains that the Mishnah means all מומחין – experts in halachos of shechitah may shecht לכתחלה. מומחין ואף על פי שאין מוחזקין – This applies to all experts, even though they are not established as competent in the skill of shechitah (i.e., that they do not grow faint and pause during shechitah). The definition of a מומחה is שיודעין בו שיודע לומר הלכות שחיטה – that they know about him that he is able to relate the laws of shechitah. If this is not known about him, he may not shecht without supervision, but if he did, we test him; if he can recite the laws of shechitah, the meat may be eaten, but if not, it is forbidden (this is the meaning of the Mishnah’s second phrase). In Ravina’s second interpretation, the Mishnah means that all מוחזקין - established as competent in the skill of shechitah may shecht לכתחלה. This applies to all מוחזקין, even if they are not known מומחין (because we assume most people who shecht are מומחין). The definition of "מוחזקין" is one who shechted before us two or three times and did not grow faint. Otherwise, he may not shecht without supervision, but if he did, and said ברי לי שלא נתעלפתי – “I am certain I did not faint,” his shechitah is valid.